a few days ago, i bought call of duty 4 off ebay and i just got it today.  my question is with the multiplayer aspect.  my cousin said he'd explain it to me but i haven't been able to get a hold of him.  basically, what is the whole ranking system and perks and levels and stuff in the game all mean.  i have to wait about a week before i can play the game because my 360 is still at home.  if someone could kind of give me a breakdown of what it all means, that'd be great.

Basically, when you kill people, or achieve objectives, you get XP and this levels you up. As you level up you unlock new weapons and perks, which give you things like faster reloading, more health, more grenades. There are also challenges which can give you XP and unlock new skins for your guns, so if you get 150 headshots with the AK-47, you get a golden gun