To support the service (and keep free features free), we sometimes run advertisements, and the agreement for this has been in our terms of service since 2006.
We try hard to only run them in limited places. If you would like to completely eliminate ads from appearing on your blog, we offer the No-Ads Upgrade: http://en.support.wordpress.com/no-ads/
However, if you are seeing ads that state "ads not by this website" or ads from highlighted words in your posts, these are not from us and typically added by an infected or corrupt browser add-on or extension. If that is the case, I recommend deactivating your browser's add-ons or extensions individually until you find the cause. You should find instructions for this in your browser's support documentation.
To be clear, all of our ads are family-friendly, and several are actually just US television commercials.