To start with I would recommend the source for basic rules:
http://www.google.com/webmasters/guidelines.html
Per definition
SEO itself is a spam!
Try to think in a SE way. What would YOU consider as spam if designing a SE?
Google describes it in a simple way:
"Make pages for users, not for search engines."
and
"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."
and another one:
"You should never have to link to an
SEO" !!
However, some people are very confident that Google don't filter many SE spams even the simple ones like cloaking, etc.
GoogleGuy, the unoffical Google voice on Internet put it in this way:
GG:
"I'm happy for anyone to define spam anyway they want, or even to claim that it doesn't exist. I think Google has some pretty clear examples of what we consider spam in our quality guidelines, and people who think that there is no such thing as spam should still be aware of our quality guidelines."
Another reply to a spam question:
Q: "Would Google consider a two tier system for spam reporting? Level 1 - the current 'report a specific spammer' - where the focus is on reporting a specific (usually competitor) url. Level 2 - a higher level, formalised programme leveraging the skills of
SEO professionals - where the focus is on reporting 'spam methods' - with an aim of improving the filters and improving the results."
GG A: "I would love to see more reports of spam methods rather than just individual instances. If a picture is worth a thousand words, then reports of spam methods can be almost as valuable in the same way. Since we're most interested in working on scalable algorithms, it helps to have descriptions of methods rather than pointing out a specific instance of spam."
This was some time ago. Clearly, they didn't have fully automated algo for detecting spam.
Did they finally made it recently?
Only Google knows.
For additional info you may look at:
http://www.sethf.com/anticensorware/...esian-spam.php
http://www.benedelman.org/spyware/whenu-spam/
http://www.webproworld.com/viewtopic.php?t=19333
etc., etc...