Black Hat SEO are unethical technics that break the search engine's rules and regulations in order to get higher page rankings.
Any method that is seen as being deceptive is classed as being Black Hat SEO. A site can be blacklisted for using these techniques. BMW was blacklisted by google for a short period in 2006 for using doorway pages. Another company was blacklisted for purchasing backlinks and subsequently lost millions.
These are just two examples but the search engines blacklist numberous site for using dubious SEO techniques.
One of Google's statements on Black Hat is:
If a SEO creates deceptive or misleading content on your behalf, such as doorway pages or 'throwaway' domains, your site could be removed entirely from Google's index. Ultimately, you are responsible for the actions of any companies you hire, so it's best to be sure you know exactly how they intend to 'help' you.
These are some techniques that you should definately avoid using:
Using white text on a white background (or any text that is the same color as the background)
Including text behind an image
Using CSS to hide text
Setting the font size to 0
(Agent Name Delivery)
Presenting different content to search engine 's crawler than to user's brower. This is done showing content based upon IP address or the User HTTP. There is a server-side script that identifies the user and gives the crawler different content.
Doorway or Gateway Page
A doorway page often does not have any content that relates to the overall purpose of the site.
The only purpose of a doorway page is for it to
rank high with the search engines. It either links or redirects the user to another page.
Website Mirrowing - websites with identical content
Translation into other languages is considered acceptable
Duplication - Republishing information from other sites without giving a back-link and credit to the orginial article.
One of the main aspects of Panda's ranking system is to penalize sites that mainly republish articles.
Repetition of content often in E-Commerce or CMS sites. This can be fixed by suppling a canonical URL
Placing links where the users cannot see them in order to increase link popularity.
Overloading the page with keywords.
Submitting your site to link farms, which are mainly sites that exist for the sole purpose of creating back-links to sites (example of a company that was blacklisted.)
Meta Tag Spam
Loading the meta tags with keywords that have nothing to do with the site. This is one of the reasons that Google stopped using the keyword meta tag in its page ranking.
A Throwaway Domain Is a domain that has very little value as a site in-and-of-itself. It is often used for cloaking and/or back-links.