Black Hat SEO
Search Engine Terms of Service
Firstly, a fore note. Search engines have terms of service and many of them actively ban web sites that do not comply with their terms of service. Black hat SEO is essentialy a manipulation of the search engines algorithm and spam. In the SEO industry black hat SEO is looked down upon and seen to be unprofessional. There is nothing illegal about it, its just unethical.
Black Hat SEO
So what exactly is black hat SEO? Black hat SEO usualy involves some form of excessive useage of keywords or links or something else a bit decietful. A common black hat technique is blog spamming. This is where the black hat SEO, or very often an automated program goes and posts comments on forums and blogs that are completely unrelated to the topic just to get a link. Many forums and blogs get around this by using the nofollow attribute on the href element.
Another common black hat practive is keyword stuffing. Pages with excessively high keyword density, URLs with the keyword in several times, lots of h1, h2 tags etc with repetative keywords. Very oten the output of this code is hidden or the same colour as the background.
Black hat SEO can give you good results quickly but they usualy don't last and will often get your site abnned or heavily penalised. A lot of black hat SEO tecnhiques use redirected pages which, once you stop paying for the service, stop redirecting.
Many Black Hat SEO companies say they can guarantee you results and claim to be in some way affiliated with search engine companies. This is pure fiction! Black Hat SEOs deliberatly tempt clients into going for keywords that will be of no use. A #1 ranking is no use to anyone if there are 0 searches per month for that term.