Black hat:- here you can do all this things keywords stuffing Hidden or invisible, unrelated text Meta tag stuffing "Gateway" or doorway pages Scraper sites Link farms Hidden links Sybil attack Spam blogs Page hijacking Buying expired domains Spam in blogs Comment spam Mirror websites URL redirection Cloaking Result: become penalize and banned from google
Yup, Aryans is right, The process which you follow for optimizing websites against Google webmaster Guidelines called as black hat.
In Short Black Hat SEO is nothing but just Unethical ways of achieving high PR and High SERP result .. or a trial to manipulate the Search Engine Ranking by Unethical Ways... If you found guilty of Trying a Black HAT Seo technique then Search Engines may Bann your website.
this wikipedia page defines black hat and compares it with white hat seo. a good resource http://en.wikipedia.org/wiki/Search_engine_optimization