Hi everyone,
Black Hat SEO covers many different aspects of website structure and methods of online promotion. It has its place for some industries which have a limited marketable time frame but each person needs to consider the consequences of using such techniques. It's not illegal but your website can be blacklisted by search engines.
A simple example of what is considered black hat would be to include many similar keywords on your home page and make them the same color as the background so they can't be seen by humans but can be by search engine spiders. This method is old now and is very easily picked up by Google and other search engines.
Basically Black hat SEO is anything against the terms of service of the different search engines, Google being the main one.
The best place to get a true indication of black hat techniques visit fantomaster.com as they are the world leaders in black hat techniques.
Hope this helped.
Cheers