A Speedhack is a single program or a codestring packed into a multihack. It makes you move and attack faster. BUT: You need a fast Processor to get decent running/attacking speeds because the speeding depends on how fast your CPU is and it isn't helpful in Singleplayer games, because everything (enemies) gets faster.
Speedhacks are commomly used in counterstrike to piss tarded CS players off. Those hacks are very rare in other games.
If this hack is combined with an aimbot, it's the ultimate solution to ruin the average cs nerd's fun, ergo his complete life.