According to Wikipedia ...."Black Hat SEO or spamdexing, use methods such as link farms and keyword stuffing that tend to harm search engine user experience. Search engines look for sites that employ these techniques and may remove their listings..."

Here's a list of a black hat SEO techniques (e.g. keyword/meta tag stuffing, doorway pages, link farms, wiki spam, spam blogs, etc)  that should be avoided

http://en.wikipedia.org/wiki/Black_hat_seo

cheerios :P