There was once a movie by Roland Emmerich, involving the US military travelling to distant planets through the use of an ancient wormhole device and meeting new civilizations; this film and that device were, known as Stargate.
Taking cue from the movie, the show tried it's best to include hammy over-the-top villains who use mythology and religion to solidify themselves as deities.
But they couldn't have a bunch of Greys who use human hosts and are just a bunch of pricks (X-Files had that covered).
So they looked over a couple of Star Trek: TNG episodes and found the idea of slimy,wormish, parasites that can massively increase a human's strength and what not (the episode was called: The Conspiracy, in case you were wondering).
But worms just aren't evil enough so they watched over a couple of nature shows and said "lampreys are freaking evil! Yeah let's make these parasites out to be gross, alien lampreys!". So they dubbed these alien-lampreys; the Goa'uld, and had them act like a lot of mad rulers who are utterly insane.
But these Goa'uld were usually bickering over territory, slaves and oh the usual stuff that megalomanic, deluded, false-gods have to deal with. After a few seasons the show, the creators had to get someone else who could be more than just a bunch of self-conceited models in drag.
Sexy, evil, alien. chicks. Confirmed!
So once again it was back to the Star Trek: TNG archives to pull out a bunch of machines who are on a single minded goals of reproducing and assimilating; the replicators. Well they were just a bunch of mechanoid insects who went around eating stuff and generally being a nuisance. Eventually like a lot of evil alen bad guys, they had to human face. Which meant the replicators became a bunch of humanoids who could only brag at how organic life completely sucks, forgetting of course that imitation is a form of flattery. Anyways the replicators and the Goa'uld eventually went at it, with the replicators coming up on tops. In the end, SG-1 some secret alien do icky to win the day.
"How and why would androids swea...", "Shush! my imagination is working!"
With two Trek inspirations for their villains, they needed to go 3 for 3. But having watched TNG, they realised that Picard tended to beat his enemies with reason and logic and thus they needed aliens that generally piss-off the well-cultured Picard. So in other words; the Q.
Well not totally like the Q; but a bunch of stagnant, highly-evolved, alien beings that love to mess about humanity all the same. But by this time the creators had been flicking through DS9 and discovered the Pah-wraith. Thus they created the Ori. The Ori were a bunch of energy beings who manifest themselves as flames, and are at war with their more reasonable brethren. When they weren't messing around in mortal affairs, they were battling their counterparts, in another dimension.
Too easy? I guess some Mr Freeze jokes might be neccessary
Oh yeah, there were human bad guys who were part of some shady government arms and of course some wealthy businessmen. Oddly enough, they're scattered but still around. Probably because they realise the best defence is legal one involving loopholes and out-of-court settlements. If only the aliens were as smart as to use mundane bureaucracy and red-tape to stop the good guys.
Like all great evil politicians, he became president in an alternate timeline.
Well the US air force had to pick out the best of the best. So that obviously entails hiring: Col Jack O'Neill, Dr Daniel Jackson, Captain Samantha Carter and Teal'c.
Col Jack O'Neill
Col Jack O'Neill was the obvious choice as leader, seeing as he was manically suicidal in the original movie, but at least he has experience. He seemed to have lost a couple of pounds of muscle, wears brown contact lenses and he quit smoking. Now, he uses humour in the most inappropriate situations and makes a lot of outside references to cleverly confuse his bedazzled foes. Hobbies aside happen to be TV, beer and fishing.
He's got a lot on his plate
Dr Daniel Jackson
The US military needed someone who could interpret foreign languages and communicate with the already English-speaking aliens, so they brought back Dr Daniel Jackson. He happens to be the moral compass of the team who argues a lot against Col O'Neill, mainly because he knows he can't face a court martial for disobeying order. He happens to get a lot of female attention despite fact that he is tracking his abducted wife across the galaxy; the universe must REALLY hate fidelity. Also, he happens to die a heck of a lot, although he always comes back. The creators brought in some replacement for like one season and then he came back naked and with amnesia in what could have only been, one of the greatest hangovers in human history. Despite being a civilian he eventually gets something other than some 9mm, so he gets the best of both worlds.
So there are two other guys; Capt Carter and Teal'c.
Capt Samantha Carter
Capt Samantha Carter
Capt Carter is a genius who was going to be astronaught but went on to follow the career of her father. Her job was to give out the technical jargon and wow over the potential uses of technology could be for our own, little planet. Any attempt at romance ended up with her boyfriend's getting killed, so she has as much luck as her team mates in that regard. Its implied that she had a thing for Col O'Neill but regulations and their careers kept them apart.
She's got her sights aimed high. Wants to get that achievement for most headshots lined-up.
Then there is the big man: Teal'c. Basically the heavy duty kind of guy who gets the biggest guns and just far better at killing people than his other teammates. He wants revenge against the man who killed his father and the freedom of his people from the Goa'uld. He also says "indeed", ALOT!
A firm believer in the big stick ideology.
By the later seasons, Richard Dean Anderson wanted to spend more time with his family so like all good sci-fi shows, they poached of characters from other good franchises. This time, they got John Crichton and Aeryn Sun from Farscape. This show loved to poke holes at itself so there were inevitable references to this.
John's other hand is placed elsewhere.
Daniel Jackson was briefly replaced by Jonas Quin. But nobody cared about him so the good fandom, purged their collective memories of him.
Like all sci-fi shows, they can't help but mess this up.
The show stipulates that a bunch of aliens called the Ancients created humans in their own image. Why they did this was anyone's guess, but the Ancients were super-smart except for the fact that they're all dead.
Anyways they can't help but kick the idea of evolution in the nads.