Rack Focus

A Rack focus in filmmaking and television production is the practice changing the focus of the lens during a shot. The term could refer to a small change of focus or it could refer to a more abrupt change. If thefocus is shallow than the technique becomes more noticeable. In professional films a camera assistant called a Focus puller is responsible for Rack focusing.

http://en.wikipedia.org/wiki/Racking_focus

This is a beautiful effect in many great movies where the focus remains on a specific character but the background rapidly gets closer or further away.