A model release is a legal contract between a photographer/artist and the individual(s) that is/are the subject of the photograph. By signing a model release, the individual/subject of the image gives the photographer/artist permission to distribute the image for broad usage rights (including commercial, promotional, and marketing uses, and modifying the image) without further compensation to, or approval from, the model.
Find model releases here.
See when to get a model release here.