CrackLib tests passwords to determine whether they match certain
security-oriented characteristics, with the purpose of stopping users
from choosing passwords that are easy to guess.
CrackLib performs several tests on passwords: 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; and 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. Install the cracklib package if you need a program to check users' passwords to see if they are at least minimally secure.