Originally born of a symbiotic bond between an alien and photographer Eddie Brock, Venom is one of Spider-Man's deadliest foes.
In the Mid-1980s, Marvel Comics published a crossover even that featured most of its heroes entitled "Secret War." In this story, Earth's mightiest heroes were summoned along with their arch-enemies to another world to fight in a cosmic game of what was, basically, chess. Among these heroes was Marvel's flagship character, Spider-Man. The heroes and villains soon learned that they had been summoned to do battle against each other by a being known as The Beyonder. He had brought them to a planet known as BattleWorld, in the hopes that they would... do... battle... there. Evidently, the 1980s were a time of relatively uncreative and bland naming at Marvel Comics.
At the same time, back on Earth, the B-Level supervillains who had been lying in wait didn't deem the departure of the heroes a good enough opportunity for them to take over the world, so they sat and waited with their thumbs up their asses, hoping that their enemies would return home safely.
After a battle of epic proportions, Spider-Man found that his costume had been torn to shreds. As a result, Thor directed the arachnid-themed hero to a facility where he could obtain a repaired costume. Deciding that it would be a good choice to wander around an alien planet without a guide, looking for technology he had no idea how to use, Spider-Man set out on his quest for a new costume. Instead of finding one, however, he fumbled with some alien technology and unknowingly opened a prison module containing a puddle of what could only be described as extraterrestrial goo. Initially, his spider-sense warned him of danger, but he dipped his hand into it anyway.
"This is in no way a bad idea."
Spider-Man, after putting up no visible fight to get the self-propelling creature off of his body, realized that his new, black costume could replicate civilian clothes, create an organic version of his webbing, and had physical side effects on him, such as increased strength and stamina.
None of these factors set off any alarms in his head, however, and he wore it anyway.
After weeks of using his new costume, Spider-Man noticed that, during the day, he was beginning to feel lethargic. Using his massive, science-y brain, he decided to discuss the situation with fellow genius, Reed Richards of the Fantastic Four. There, Spider-Man learned that the costume is a sentient alien symbiote that wished to fuse itself with him permanently. Doctor Richards explained that the lethargy Spider-Man felt was from the symbiote cavorting about New York City, using Spider-Man as a host. After conducting a battery of made up, comic book tests on the creature, Spider-Man, Mr. Fantastic, and the Human Torch, learned that it was vulnerable to both supersonic soundwaves and heat. Instead of using the Human Torch's ability to control fire and heat, though, the three of them decided to build a sound-based ray gun to, basically, annoy the creature off of Spider-Man.
Somehow, Reed Richards (arguably the smartest man on the planet) was unable to contain the extraterrestrial puddle, and it escaped. For whatever reason, it headed back to Peter Parker, and tried to force itself into his bloodstream. Finding his way to a bell tower, Spider-Man subjected the alien to the torturous clanging of a church bell, which was surely against some sort of intergalactic torture legislation.
Once expelled from Spider-Man's body, the symbiote, using its remaining strength, carried an unconscious Spider-Man to safety from the bells before it slithered away. For whatever reason, it wanted to save the person who just tried to kill it with a giant bell.
After trying to rape, absorb, and generally do some unpleasant things to Peter Parker, the symbiote itself lay dormant, waiting for a new host and regaining its former strength. For whatever reason, the comic book writers in charge of the Spider-Man books at the time also decided that the creature ought to grow steadily hateful of Parker for rejecting it, instead of keeping hold of the same undyingly loving emotions it had when it could have easily killed him. It makes sense, after all. Consider the symbiote as someone who had just been dumped. While, at first, it might harbor feelings of love and longing, over time it grows to steadily want to rip out the throat of the one who caused it so much heartache. Granted, in this case, the symbiote represents the percentage of the human population generally considered to be batshit insane.
For weeks, the symbiote waited in the church tower, hoping for a host to come along. For some reason, this creature that inherently requires a host organism didn't die in the weeks without something to feed off of. This, however, is a fact that the writers only tend to glance over, and can be called a "plot hole."
Enter Edward Brock.
Brock, like Peter Parker, worked at a New York city newspaper. After being diagnosed with terminal cancer, however, the reported found himself just a tad depressed. Devoting himself to his work, he eventually found an elusive serial killer, who openly confessed to dozens murders to the reporter. Elated, Brock turned the killer in to the police, only to find that the man he had been investigating was a compulsive confessor. The real killer, in a terribly inconvenient plot twist, had already been caught by Spider-Man. Disgraced, Brock was fired from his job and forced to work at trashy tabloids such as "Star," "The National Enquirer," and "The Limbaugh Letter." Blaming Spider-Man for his downfall, Brock grew more stressed about the presence of the cancer in his system, and started compulsively exercising to calm himself. Finding no relief, Brock's depression only grew worse as the days passed. His wife left him, his tan faded, and he just got utterly unpleasant to be around. In a moment of weakness, he decided that his only option was to end his life. Seeking forgiveness from God before committing suicide, Brock made his way to a local church and prayed.
Coincidentally, this happened to be the church that Spider-Man ditched his crazy alien sorta-living costume. And, as if by magic, the symbiote sensed Brock's confusion, hatred of Spider-Man, and the adrenaline in his system caused by his cancer. In a single moment, the symbiote pounced, and found a new host in Edward Brock. Feeding off of the cancerous cells in his body, the symbiote granted Brock all of the powers of Spider-Man and kept him alive long enough for him to get the shit published out of him by Marvel Comics.
And, like that, Venom was born.
Venom garnered praise from critics and fans alike. At last, Marvel had produced an evil counterpart for their most popular character. Batman, stoic and reserved, had the Joker, full of life and psychotic energy. Professor Charles Xavier, a pacifist by nature, had Magneto, a man devoted to destroying the human race. And, at last, Spider-Man, bound by responsibility, had Venom, a dark shadow of the Webhead himself, who would be untamed with his powers. Responsibility won the day, though, for after one of their first encounters, Spider-Man was able to overpower his psychotic duplicate. Brock was incarcerated, the symbiote removed from his body, and forced to await trial. The symbiote found him, broke him free from jail, and, in so doing, left a microscopic piece of itself behind, creating another symbiote. This creature bonded with Brock's cellmate, Cletus Kassidy, a union that would lead to the creation of the villainous creature, Carnage.
Gradually, as artists and writers shifted, the character developed a more and more distinctive appearance. While bonded with Eddie Brock, Venom gradually developed sharp fangs, a large tongue, and green drool that would strike terror into the fear of "Spider-Man" reading six year olds everywhere.
Venom, in all of his slobbery glory.
Of course, like every other comic book character ever written, Venom was featured in his fair share of bad, silly, and just plain bizarre story arcs. One of the most notable being the "Lethal Protector" story.
After deciding to abandon his hunt for Peter Parker, Eddie Brock moved to his hometown of San Francisco. While there, Venom was captured by a covert group called The Life Foundation, who had one purpose in holding him hostage: to force him to spawn more symbiotes. That's right. A company called The Life Foundation captured Venom, a deadly serial killer who was probably on dozens of most wanted lists, and decided to make fucking more of him. They were successful, and were able to create five new symbiotes, all of whom were as deadly and psychotic as Venom.
Mother of God, we are FUCKED. We are so FUCKED.
As if one symbiote in San Francisco wasn't bad enough. After the Life Foundation decided to get involved, there were six of these terrifying creatures.
Of course, Spider-Man and Venom were able to join forces, put aside their differences and are able to beat the shit out of all of them, eventually deciding to part ways again on the sole condition that Venom stopped committing crimes.
Venom soon came across an underground group of descendants of the disastrous 1906 San Francisco Earthquake. Literally. Brock discovered a group of hundreds of these ancestors living beneath the streets of San Francisco. These innocent squatters had been holding up various construction and digging projects in the city. So, instead of filing legislation, calling a Congressman, or just politely asking the people to leave, the owner of the construction site hired a gang of high-tech mercenaries, ironically called "Stalkers" to find and kill the mole people. Naturally. Of course, Venom took pity on the gang of outsiders and assured them he would be their "Lethal Protector," and threw himself into the middle of the fray. Spider-Man, naturally, got wind of Venom's involvement and decided to pay a visit to San Francisco, just to say "hi" and provoke a fight for no real reason whatsoever. The two eventually defeated the Stalkers, and Spider-Man left Venom to live in peace once again.
What came after "Lethal Protector" were a series of confusing story lines, all with Venom at the center. The character's history grew convoluted, complicated, and downright stupid. For instance, one story line led to the symbiote needing phenethylamine to survive. In Marvel Comic lore
After a while of Marvel deciding not to put Venom in every single Spider-Man comic ever written, the character appeared once more. The years of living with cancer had caught up to the supervillain, and the only thing keeping him alive was his alien buddy. The symbiote, evidently no longer hungry for cancer cells, decided that it no longer wanted to be bonded with Brock. After seeing an emotionally uplifting movie, Brock decided to sell the symbiote in an underground auction, with the intentions of giving all of the profits to charity.
Sort of like this, but with a psychotic alien symbiote that eats away at its host.
Also, this was probably the movie that put the incredibly stupid idea into Brock's head.
The winner of the auction was crime boss Don Fortunato, and Brock and the symbiote parted ways. After getting his ass handed to him by Spider-Man, Fortunato was humiliated and the symbiote abandoned him in mid-air after a leap across a building, sending the gangster to his death.
After years of over-publishing, fans had finally thought that Venom would be laid to rest, that the stupid storylines would stop. Marvel, however, had other plans, and a trick up it's sleeve that left people baffled...
Just when things had seemed like they'd calmed down... just when the symbiote was out and about, waiting to be summoned by another host months down the road, Marvel decided to put the symbiote on another villain of Spider-Man's. This time, it was the deadly Mac Gargan, known also as the Scorpion. After being sent to kidnap Peter Parker's Aunt May by Norman Osborn (aka Supreme Commander of the Group Dedicated to Fucking with Spider-Man's Life at Every Turn), Mac Gargan was approached by the symbiote, which had sensed his intense hatred of Spider-Man. Gargan accepted the creature's offer to bond, and together they formed the New Venom.
This must be a fucking joke.
While the stories progressed, there was considerable backlash among fans regarding the change, and the decision to change something which had been enjoyed by comic book readers for years.
Another highly popular "reinvention."
After a series of obligatory fights, moments nearly killing the damsel in distress, and a few random adventures on other planets, Gargan was most recently recruited by the government to join the superpowered team of villains known as The Thunderbolts. Outfitted with an electrical implant to keep the symbiote in check, he was seen by the public in the comic books as a hero, even being made into an action figure.
You know... for the kids!
Earning plenty of praise for himself, this Venom gradually grew more and more powerful, loving the adoration of the public. Each time he encountered a new opponent, he became a little bit more savage. In one encounter with a villain, he almost went so far as to devour his foe's flesh. Despite fearing the amount of raw control the symbiote had over him in the heat of battle, Gargan grew steadily addicted to the virtually limitless strength it provided him with. Like a drug addict, he couldn't imagine life without it, and grew more and more insane. Sort of like a certain Texan did, once he had the power of the Presidency of the United States in the 2000's.
Norman Osborn proceeded to form his own superhuman team, the Dark Avengers, and recruited Venom to join. Osborn injected the symbiote with a magic, mystery, comic-book concoction that (somehow) reduced the creature to the same size it was when it bonded with Spider-Man. When Gargan is in the heat of battle, however, the symbiote is able to increase in size once again, turning Gargan into a monster. Under Osborn's orders, Gargan now parades about New York without the ominous fangs and mouth of Venom. Instead, he has a mouthless face, emulating the appearance Spider-Man first had when wearing the costume. Osborn revealed his Dark Avengers to the public, claiming that Gargan was, in fact, the Amazing Spider-Man.
Gargan as Osborn's Spider-Man
See also: Ridiculous