A Talent Release is a contract that an actor, or the person who appears in a video signes to give full consent to being filmed. There are standard releases on-line. It’s important to make sure that the release states that you can manipulate the media in any way you want, that you can use the media for all perpetuity (eternity), and in any medium existing or invented in the future.
Most importantly: do not shoot a single frame before you don’t have a signature!
When shooting big crowd scenes, using ‘real’ people, i.e. you’re shooting in the streets of Manhattan for instance you can put up a sign saying something along the lines of: By walking down this street (or entering this store, etc), you give consent to be filmed. This can work for small independent films, but for everybody else you need a release form from every single person you see in the picture.