Crack is a small piece of program that generally is made so as to bypass all Piracy security measures of a particular software. They usually come in 3 types.
1. Direct EXE replacement:
You get a cracked exe which you replace with that of the original and your software usually acts as a paid then.
2. Patching:
A small utility patches your program's main exes and other files with some extra bytes, or in some cases, removes some bytes and makes it a full-version
3. Direct Serial/Keygen method:
Sites provide a brute-forced or exploited key or even a random key generator for specific softwares, that one can use in the software's "Enter key to unlock full version" dialog box and unlock the full version of the software.
If you wish to read on on this crucial Software piracy topic, I suggest you go to *en.wikipedia.org/wiki/Software_cracking .
Whatever you do, never google for 'crack'