This information was copied from http://www.google.com/adsense/support/bin/answer.py?answer=100557, and applies to my site because I use Google Adsense.
- Google’s use of the DART cookie enables it and its partners to serve ads to your users based on their visit to your sites and/or other sites on the Internet.
- Users may opt out of the use of the DART cookie by visiting the advertising opt-out page.
- Notify your site visitors of the third-party vendors and ad networks serving ads on your site.
- Provide links to the appropriate vendor and ad network websites.
- Inform your users that they may visit those websites to opt out of cookies (if the vendor or ad network offers this capability). Users can opt out of some, but not all, of these cookies in one location at the Network Advertising Initiative opt-out website at www.networkadvertising.org/managing/opt_out.asp.