"You retain your rights to any Content you submit, post or display on or through the Services. By submitting, posting or displaying Content on or through the Services, you grant us a worldwide, non-exclusive, royalty-free license (with the right to sublicense) to use, copy, reproduce, process, adapt, modify, publish, transmit, display and distribute such Content in any and all media or distribution methods (now known or later developed).

Tip: This license is you authorizing us to make your Tweets on the Twitter Services available to the rest of the world and to let others do the same."

Without this, all those embedded Tweets you see everywhere (like in news stories) wouldn't be available (this is why I think these services should all give you the ability to disable embedding your post(s) like Flickr/YouTube do. It would make everything much clearer and more straightforward.)