Black Hat SEO

The opposite of White Hat SEO (ethical search engine optimization), Black Hat SEO methods go against search engine guidelines. Ultimately all sites utilizing Black Hat methods will be penalized, and in many cases web sites will get banned by the search engine altogether. Examples of Black Hat SEO techniques include, but are not limited to:

Keyword Stuffing.
Repetition of keywords with the goal of increasing the keyword percentage past that of the competition. It's not a game of "whoever has the most keywords, wins" - search engines have sophisticated text recognition software that flags unnatural repetition and keyword lists.

Doorway/Cloaked Pages.
Any tricks that get search engine spiders to follow links or see content not actually associated with the target site, such as cloaking, 302 page hijacking, or redirects, will result in the site getting penalized.

Hidden Text.
It's a ploy that started in the 1990's, but amazingly, some people still think they can get away with it. A webmaster adds text, usually at the bottom of a page, the same color as the background color of the page, making it invisible to human eyes, but readable by search engine spiders. Spiders can interpret HTML code and easily see what's been done. Your site probably won't get banned for this, but it's highly unlikely it will ever rank near the top of a search engine's result pages for a search query.

Copied/Scraped Content.
Sometimes people think one listing in the top 10 search results is not enough, and that if they have 5 sites ranking well, their chances of getting a hit is five times as likely. The problem is that search engines can detect sites under the same company umbrella, and while that isn't against rules, copying information site to site is. "Scraping" content from other sources is also a useless practice (duplicate content is frequenty filtered out of search results) and can even result in copyright infringement.

If you aren't sure if your SEO methods are White or Black Hat, take some time to familiarize yourself with search engine guidelines:

Google Webmaster Guidelines
Yahoo Webmaster Resources
Bing Search Guidelines