Comp is short for "Competitive", a game mode in Counter-Strike: Global Offensive. Two teams of five play as terrorists or counter-terrorists and compete in a best of 30 rounds where the terrorists attempt to plant a bomb while the counter-terrorists attempt to repel the terrorists away from various bomb sites.
Players are ranked based on an elo system, from Silver I to Global Elite. It is every counter-strike player's worst nightmare to be placed in Silver I.
A: Hey bro, wanna comp?
B: Not now man I don't want to lose my rank because of you
short for "graphic compositor"... word used to distinguish between those who think they're in the printing/graphic arts industries and those that really are... the journeymen. In true speak these people would now be prepress operators or similar