Copyright - a set of exclusive rights regulating the use of a particular expression of an idea or information.

http://www.photolaw.net/index.html

Stock photography - consists of existing photography that can be licensed for specific uses.

http://www.sxc.hu/

Royalty-free - describes material (typically graphics such as stock photography and icons, but also sound such as music loop samples) that may be used for profit, without paying royalties.

http://www.callofthewildphoto.com/

Model release - known in similar contexts as a liability waiver, is a legal document typically signed by the subject of a photograph granting permission to publish the photograph in one form or another.

http://www.dpcorner.com/all_about/releases.shtml