The dolly zoom, also sometimes referred to as the "vertigo zoom" is an in camera effect that seems simple on the surface but actually requires a great deal of talent and practice to pull off. It's also perceived by some as a little gimmicky so it's only used when it's really called for.
When is it called for? Well, that's why we have a video to explain it to you.
And, for your viewing pleasure, here's a montage of dolly zoom clips from various films.