a program that modifies another program
a program to ignore the relevant encryption-modules of the original versions
a small program intended to defeat software's copy protection, thus allowing unlicensed copies to be made - stealing it, in effect
a small utility designed to eliminate whatever protection has been placed by the computer manufacturer on a shareware program
A crack is a small program/file whose typical purpose is to make the trial version of a program into a full version. It might do this by disabling any time limit that the trial version includes. As with 'warez', this is illegal.
A tiny executable file that is used to transform a shareware program into the full version. Also used to remove any copy protection from the main executable of games (this will already have been done in "ripped" warez games).