They zoom out while simultaneously moving the camera forward. Or you can do the opposite. It’s called the “Hitchcock Zoom” since he used it in the movie Vertigo so simulate the feeling of actual vertigo.

There are many names for this move, but essentially it’s a camera (lens) optical zoom while a dolly pushes the camera rig forward on a track.

On a push/zoom (the typical horror effect, e.g. in the hallway scene in the first Poltergeist film), what’s happening is the camera rig sits on a dolly, which is pushed on a track toward the subject, and (whether done manually or with the assistance of a rigging/tracking system) the camera will zoom out while the dolly pushes forward, keeping the subject in the center of the frame.

Here’s a useful link that describes some (very technical) approaches to achieving this effect:

http://www.cinematography.net/Camera_Track_While_Zoom_Out.htm

a.k.a. “Dolly Zoom” according to wikipedia; jballou is right as far as crediting Hitchcock with the move…

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