im going to summerize it as this is something very massive, i might not actully mention any names to say.lets start with basics.
Metal Gear Solid(btw, was originaly named Metal Gear but soon 3D gaming came in so Solid was placed to represent "3D")is about the destruction of a weapon. The weapon is named Metal Gear. it was first created by Octogon (Ya, the guy that helps you with that small stuff robot) which was a "REX" type. This was a cross of Infentry and artillery as infentry can go anywhere and artillery can shoot massive shells (Exact opossite for opossite type ETC:Infentry fail at weapons)so Octogon made REX. REX was capable of shooting nuclear warheads without being detected (shoot and forget) and can be shot anywhere at any distance.Long story short, Snake destroys REX, badguys find plans and replicate REX and modifyed it (Thus having diffrent versions),America make another Metal Gear (RAY) which is better then REX, destroying it but RAY has no Nuclear capabilitys. PMCs make their own Metal Gear (Gekko: thos robots that make cow noises? which i lol at) to use against war which is where the main area is which is what your playing as.
any specific question il see if i can answer (maybe not names but i could look it up 4 you)
btw, any idea why Gekko "spues" gas ontop of a body it just killed?