Space Marines are steroid infused, armor plated soldiers that kill things, usually aliens. They are very good at this.
Origins of the Space Marines
Galactic marines were mentioned in the 1939 novel Lensman. However, due to a Cracked Staff rule about reading books (don't), we never read it. Quick, Look that way!
What were we talking about again? oh. right!
The idea of Space Marines as they are today were first proposed in Robert Heinlein's 1959 novel, Starship Troopers. A few minor references in his other works had taken place, but none fleshed out the idea past a namesake. The 1959 novel featured fully powered armor suits capable of "breaking through the wall as if it was on like Donkey Kong, but are dexterous enough to allow them to softly fondle sweater-melons" [Citation Needed] But these were called "Mobile Infantry", and were equiped with tiny nukes. Their next incarnation would be one most people (and all aliens in the galaxy) would recognize (and fear).
In 1988, Warhammer 40k released hordes of angry Space Marines into the world.
No, seriously, these are the GOOD guys.
These Marines are the posterboy of the 40k Tabletop board game, and subsequent PC game based on the series, Dawn of War. They are genetically enhanced to be superior to normal humans in every way, from bone density to anger issues. Depending on the author, they can rip apart a tank and have buildings fall on them and walk away without a scratch. They zealously serve the Emperor of Mankind, and kill anything that isn't human. They aren't really considered human anymore either, but it would be unwise to press them on this.
The next stage of Space Marines borrowed heavily from the 40krines, The Starcraft Marine.
Giant shoulderpads? Check. Barrel bigger than your face? Check. Durability? eh, I'm sure someone will remember. Let's call it a day.
Reduced to tier one cannon fodder, they still made it look good. Hardened crimals forced to fight in wars against aliens and smarter aliens they didn't understand, they maintained the can-do "if it bleeds you can kill it" attitude as set by their forefathers.
All space marines can be neatly filed into these cookie-cutter archtypes.
Type I, the "Everyman" Space Marine.
First propelled by Doom in 1993, This was just a testosterone-fueled Chainsaw-wielding maniac. That being said, he holds the unique honor of being the only space marine that singlehandedly destroyed hell, and launched rockets into Satan's brain.
Yes, it actually says that.
This type of Space Marine is armed only with a paper-thin suit of armor, given a shotgun, and is kicked out the airlock. These are the closest analogue to actual Marines that we have today. They are human, with no dna splicing or anything fancy. Their armor may or may not deflect the smallest of enemy fire. With only their massive brass balls to guide them, they RIP AND TEAR YOUR GUTS.
You really just need to start taking our word on these kinds of things.
Other examples include:
Type II, the better-than-you-at-everything Marine.
First found in 40k, they have expanded to include such hero's as the Spartan's from Halo. They are trained from children to be greater than everybody at killing stuff. And it shows. For example:
WRONG CLIP! WRONG CLIP! Change it before they notice and this goes live!