Black hat SEO attempts to improve rankings in ways that are disapproved of by the search engines, or involve deception. One black hat technique uses text that is hidden, either as text colored similar to the background, in an invisible div, or positioned off screen. Another method gives a different page depending on whether the page is being requested by a human visitor or a search engine, a technique known as cloaking.
Here's a revision considering techniques: Blackhat 1) Cookie Stuffing Without Adverts 2) Clickjacking Paid Traffic Sources for Income 3) Hacking 4) Cracking 5) Licensed Software Sharing 6) Reverse SEO 7) Server attacks. Greyhat 1) Doorway pages 2) Desitation cloaking 3) Referrer blanking & spoofing 4) Keyword Stuffing 5) Mass link building (same anchor or rotating anchors) 6) Cookie stuffing with adverts. Whitehat 1) Full compliance with all parties' terms of service.