IMPORTANT: This article only gives overview of the Google policies for Adsense. You must read the official policies at the following link: https://www.google.com/adsense/support/bin/answer.py?answer=48182
Google is very strict with implementing the policies. For success with the program it is a must to keep yourself up to date with the policies and follow them.
Invalid clicks and impressions
Never click your on ads by any manual or automated methods.
Never encourage your visitors for clicks. Leave the ads alone. Let the visitors click the ads if they are interested.
You should not have content that violates the content guidelines. Some examples include adult content, content that encourages racial violence etc.
AdSense publishers may not display Google ads on webpages with content protected by copyright law unless they have the necessary legal rights to display that content.
Never modify the code provided by Google.
Google wants their ads to show up on quality websites. Websites should have quality content and should be easy to navigate. Sites that redirect users to unwanted websites or initiate unwanted downloads, contain melaware or unwanted pop-ups will be banned.
Competitive Ads and Services
you should not mix the Google ads with other ads that are formatted in the similar way. you may sell ads directly on your website as long as they should not be confused as Google ads.
Google Advertising Cookies
Number of Adsense units on a page
Only upto three adsense units can be placed on a page.
Adsense search box
only up to two AdSense search boxes can be placed on a page.
For detailed official Google policies please visit this link: https://www.google.com/adsense/support/bin/answer.py?answer=48182