Black hat
SEO refers to the numerous techniques considered unethical and illegal by search engines. A website can be punished and served a penalty notice for using these SEO techniques. So be wary about using black hat SEO techniques and if you aren't sure what they entitle, here's a list to help (Please feel free to add to it, if I miss something)
1. Hidden text /links
2. Duplicate content
3. keyword stuffing
4. Alt tag spamming
5. Meta tag stuffing
6. Title tag spamming
7. �Gateway� or doorway pages
8. Cloaking
9. Article Spinning
10. Link Farm
11. Purchasing expired domains
12. Spam in comments
13. Mirror websites
14. URL redirection
15. Hitting trackbacks