It's called Dolly Zoom (but there are a couple of alternate names for it)
A dolly zoom is a cinematic technique in which the camera moves closer or further from the subject while simultaneously adjusting the zoom angle to keep the subject the same size in the frame.
The effect is that the subject appears stationary while the background size changes (this is called perspective distortion).
Invention of the dolly zoom is credited to cameraman Irmin Roberts.
The technique was made famous by Alfred Hitchcock (Vertigo being the best-known example), and was used by Steven Spielberg in Jaws and ET.
The dolly zoom is commonly used by filmmakers to represent the sensation of vertigo, a "falling-away-from-oneself feeling" or a feeling of unreality, or to suggest that a character is undergoing a realization that causes him or her to reassess everything he or she had previously believed.
After Hitchcock popularized the effect the technique was used by many other filmmakers, and eventually became regarded as a gimmick or cliché.
This was especially true after director Steven Spielberg repopularized the effect in his highly regarded film Jaws, in a memorable shot of a dolly zoom into Police Chief Brody's (Roy Scheider) stunned reaction at the climax of a shark attack on a beach (after a suspenseful build-up).
Examples on YouTube: