It is quite simple.
Google will start bashing sites down.
The sites that will go down are sites created to make money in the bad way.
1. Affiliate sites filled with links but no content.
2. Sites with black hat SEO which fool the search engines to think they have actual value.
3. Sites with too many links pointing to them, but low, poor or no content.
4. Sites with malware.
5. Sites with bad redirects which seem legit but redirect you to sites which are not.
It's all in the Google Webmaster's guidelines.