The free program WinRTGen can be used to generate rainbow tables. Rainbow tables provide the source for a hash value, and are helpful in cracking passwords. They are in effect reverse lookup tables.
For security purposes passwords are usually stored as hash values in databases. A rainbow table will give plaintext phrases which correspond to large numbers of different hash values.
A rainbow table approach to password recovery differs from a brute force attack in that it uses storage (a large data bank of passwords) rather than processing power. It's important to keep in mind that WinRTGen may take up 500 MB or so to store a rainbow table.