Cloaking. Don't cloak! Cloaking refers to the practice of telling the search engines one thing and showing people another. So for example, you write code that makes Google think it is sending people to a United Nations charity website, when in fact your site actually displays information about Canadian Rolexes for sale. Very bad.
Hiding Extra Copy. Also known and "keyword stuffing", this practice involves adding TONS of keywords to your pages - and (not so) cleverly hiding them using a white font on a white background. Sophisticated computers at Google have been on to this one for awhile.
Using your competitor's good name. Once upon a time, companies could leverage the good name of their competitors and steal their traffic. But now, loading up your HTML with the names of your rivals is the fastest way to upset your own customers. It's also a fast-track to a lawsuit.
"Borrowing" Content. This one should be obvious, but don't steal content from other sites. If you see a highly-ranked site and think "Hey, I'll use their content to raise MY rank," then think again. Google will catch you. And ban you. And heap shame on you.
With SEO, as with life, there are no shortcuts. Do things the right way and stay on Google's good side (they've got a bit of money you know).