An Over The Shoulder shot is basically what the name says. The camera is positioned with an actors shoulder in the foreground, out of focus. These can be time consuming to film correctly. The picture and video below shows an Over The Shoulder shot from the movie.
The shot is an important techniques that all filmmakers should learn, as it is narratively essential in many cases. It helps draw the viewer in by creating a sense of intimacy. The key to making the shot work is getting the right amount of shoulder in the shot. it can take practice, but can pay off in the long run.