Google Rules part 1.
A few information from firsthand, many peoples regrettably neglect its consideration.

By Google
http://www.google.com/support/webmasters/bin/answer.py?answer=35772

What are Google's quality guidelines?

These quality guidelines cover the most common forms of deceptive or manipulative behavior, but Google may respond negatively to other misleading practices not listed here (e.g. tricking users by registering misspellings of well-known websites). It's not safe to assume that just because a specific deceptive technique isn't included on this page, Google approves of it. Webmasters who spend their energies upholding the spirit of the basic principles listed above will provide a much better user experience and subsequently enjoy better ranking than those who spend their time looking for loopholes they can exploit. If you believe that another site is abusing Google's quality guidelines, please report that site at http://www.google.com/contact/spamreport.html. Google prefers developing scalable and automated solutions to problems, so we attempt to minimize hand-to-hand spam fighting. The spam reports we receive are used to create scalable algorithms that recognize and block future spam attempts.

Quality guidelines - basic principles

• Make pages for users, not for search engines. Don't deceive your users or present different content to search engines than you display to users, which is commonly referred to as "cloaking."
• Avoid tricks intended to improve search engine rankings. A good rule of thumb is whether you'd feel comfortable explaining what you've done to a website that competes with you. Another useful test is to ask, "Does this help my users? Would I do this if search engines didn't exist?"
• Don't participate in link schemes designed to increase your site's ranking or PageRank. In particular, avoid links to web spammers or "bad neighborhoods" on the web, as your own ranking may be affected adversely by those links.
• Don't use unauthorized computer programs to submit pages, check rankings, etc. Such programs consume computing resources and violate our Terms of Service. Google does not recommend the use of products such as WebPosition Gold^TM that send automatic or programmatic queries to Google.

Quality guidelines - specific guidelines

• Avoid hidden text or hidden links.
• Don't employ cloaking or sneaky redirects.
• Don't send automated queries to Google.
• Don't load pages with irrelevant words.
• Don't create multiple pages, subdomains, or domains with substantially duplicate content.
• Don't create pages that install viruses, trojans, or other badware.
• Avoid "doorway" pages created just for search engines, or other "cookie cutter" approaches such as affiliate programs with little or no original content.