Black Hat SEO & SEM Practices
Black Hat Methods
Keyword stuffing is probably the most common technique, it includes adding text/copy the color of the background, hidden on layers, or via an invisibile div.
Cloaked pages provide very different results depending on whether a person or search engine bot requests the page. Doorway pages are very low-quality sites with very little content related to what you were looking for, but are filled with hidden keyword text. Most of these sites use the 'click to enter' technique.
Link farms are groups of pages and sites that continuously link to each other to increase rankings. They're known by the legit SEO & SEM companies as "Mutual Admiration Societies."
Spam blogs sole purpose is to try and convince you to buy an affiliate product. They look like real blogs with news, information, reviews etc., but are completely fake. Common ones are for products such as diet aids (Acai) and "moms-working-from-home" get rich schemes.
Grey Hat Methods
While not exactly illegal practices, you are still running the risk of having your business website penalize or banned. The most common example would be a higher keyword usage then normal on a site. Major companies (or their marketing companies) have been caught using Black Hat methods including JC Penny, BMW and Ricoh.
SEO squatting is buying up expired domains that are relevant to your keywords and then adding a few pages of content with links to your main site.
Linking yourself is when you are posting comments on many blogs and linking back to your website.
Duplicate content on different websites - all relating to the same topic/product - for your business.
Redesigning the look of your website at very regular intervals but not changing or updating the actual content.