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).