Those are magicians you can hire with different color hats on Seriously, It's different methods of SEO, the right way and wrong way. The black hat way is the wrong way, the white hat way is the right way. There are tons of articles on this subject, I suggest you google " black hat seo" or "white hat seo". You'll find everything you need to know and then some.
Black hat is a method of getting lots of traffic, but usually results in a banning of the site. You use these methods on sites that you don't care for and don't mind getting banned by the search engines. White hat is the complete opposite. You within the guidelines the search engines have setup for sites and follow them strictly. Grey hat obviously a little bit of both. Personally, I use tactics that get traffic and results. I'm all across the board, as it's good to know black hat tactics if you need to get a decent amount of traffic quickly or for other client reasons. Do what works. Do what gets results.
White hat - "Good techniques" used by SEOs, with proper link management, good content, and a very clean optimization Black hat - "Bad techniques" used by SEOs, uses link farm, keyword stuffing, cloaking and spamdexing. Grey hat - Uses both sides, using some bad methods but sometimes can be good. In short, colors are obviously classified if the technique is bad, good, or neutral.