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