The difference is that a hacker is someone that uses their extensive knowledge of computer logic and code for malicious purposes, while a cracker - looks for back doors in programs, and exploits those back doors. Cracking is generally less harmful than hacking. Hackers are usually involved with web related hacking, like MySQL interception, or phishing, other forms of hacking would include things like brute force, or password lifting.
From @jenishtalaviya