CrackLib tests passwords to determine whether they match certain security-oriented characteristics. You can use CrackLib to stop users from choosing passwords which would be easy to guess. CrackLib performs certain tests:

It tries to generate words from a username and gecos entry and checks those words against the password;
It checks for simplistic patterns in passwords;
It checks for the password in a dictionary.

CrackLib is actually a library containing a particular C function which is used to check the password, as well as other C functions. CrackLib is not a replacement for a passwd program; it must be used in conjunction with an existing passwd program.