Black hat is a widely ranged SEO methods to improve your ranking by tricking search engines. The most famous black hat method is called "cloaking" I actually typed a very long explanation on what cloaking is right here http://forums.digitalpoint.com/showthread.php?t=941272 Hope this helps you out.
I've seen so much incorrect information on here that I decided to launch a blackhat forum and site. Please feel free to check it out at http://www.blackhat360.com