http://blog.twitter.com/2009/09/twitters-new-terms-of-service.html

Advertising—In the Terms, we leave the door open for advertising. We’d like to keep our options open as we’ve said before.

Ownership—Twitter is allowed to “use, copy, reproduce, process, adapt, modify, publish, transmit, display and distribute” your tweets because that’s what we do. However, they are your tweets and they belong to you.

APIs—The apps that have grown around the Twitter platform are flourishing and adding value to the ecosystem. You authorize us to make content available via our APIs. We’re also working on guidelines for use of the API.

SPAM—Abusive behavior and spam is also outlined in these terms according to the rules we’ve been operating under for some time.