What is Web Spam?

I am new to this forum as well as SEO. I have heard people use the word Web Spam very often. What the hell does it mean? What is the threshold limit of a site? And if a site crosses this limit, does it amount to spam? Also, what about affiliate websites? When do they come in the scenario? I think I am receiving various kinds of messages from Google. I want to get it all right. To the best of my knowledge, web spam involves repeating irrelevant phrases to manipulate your search engine ranking. Am I right?

Spam on sites can take many different forms and it is more complicated then just repeating irrelevant phrases.

Spamming keywords on pages especially when unrelated to the main content of the page can be seen as spam.

Having too high a keyword density can be seen as spam even if the keywords with the too high a density fit within the theme of the page.
http://en.wikipedia.org/wiki/Keyword_density

Keyword stuffing will be seen as spam. Googles definition:
http://www.google.com/support/webmas...n&answer=66358

Rick
Universal4

You might as well want to visit the wikipage for spam for a complete understanding as well as learn the list of activities that results to spamming the web - http://en.wikipedia.org/wiki/Spam_%28electronic%29