I don't use the word "SPAM" for very good reasons. But here's a list of things that are very high risk and that you probably should think fifty times before doing - particularly if you're new to seo.
Before I go through these there's an important point to make. Many of these things you will see in the index and might be thinking if it works for them then it's fine. Whilst it's true that it may work for them, the search engines tend to knock pages out in batches (so they might, for example, knock out all cloaking pages one time and all hidden links pages another). These are high risk strategies, which is another way of saying you'll probably get kicked out of the search engines at some time if you use them
1. Cloak - most search engines don't like it. And yes, it's relatively simple to find out.
2. Hide your links in any way - that includes 1x1 hidden pixel links and links that are the same color (or near to the same color) as the background.
3. Submit your site time and time again, just because it isn't in the search engine yet - there's probably a good reason it's not.
4. Submit your site to 809,000 search engines for just $xx.xx. There are only a few search engines worth bothering about, you'll see very little or no traffic from 808,990 of those. Once you've submitted your site to a few of the bigger directories, the bigger crawling engines will find you anyway.
5. Hidden text or text nearly the same color as the background - if the engine's don't spot it, someone will dob you in.
6. Participate in link exchanges just to increase rankings
7. Submit your site to as many FFAs, Link Farms and Guestbooks as you can
8. Use software to query the search engines and get your rankings - they don't like it. Do it manually, it's safer.
9. Use software to generate doorway pages automatically.
10. Use really tiny text on your pages
11. Stuff in keywords as much as possible, at any opportunity.
12. Try to sell the page or site as something it is not.
13. Use redirects to provide the search engine with one thing and the visitor with another.
14. Subvert the alt tag just to stuff in some extra keywords - use it for what it is meant for.
15. Copy somebody else's page virtually word for word - you won't get the same rankings anyway.
16. Bait and Switch (i.e. give the search engine one version of the page and then change it when it gets in).
17. Link to a site that's employs these high risk techniques.
18. Follow the advice of self-proclaimed "experts" who can't provide reasoning for their advice.
19. Use Javascript to try to manipulate Google's PageRank.
20. Use extensive keyword repetition in the meta tags.
21. Use many keywords in your meta tags that don't occur in the text.
22. Put multiple instances of the TITLE or meta tags on your pages.
23. Use layers and position them off-screen.
24. Use css to create invisible or very tiny text.
25. Cross-link sites excessively.
26. Misuse/Borrow competitor names/document titles.
27. Throw up a few more sites with the same content so you have a better chance of ranking.
28. Trust an SEO who guarantees you top 10 rankings.