An even more horrible special case of this process is what we might call "scan-but-don't-bother-to-pan". That is, transfer a widescreen movie to 4:3 format as Mischa describes, but leave out the panning part, just scale the image up to the full height of the TV screen and cut off the parts that end up outside the screen, without worrying about what this does to the current scene. You can often encounter this when you pick up cheap DVD or VHS releases, or in the form of quick hack jobs for late-night cable broadcasting.
The worst example I have encountered so far is in the Scanbox DVD release of David Lynch's film Blue Velvet. This movie is originally shot in 2.35:1 aspect ratio. Can you imagine what happens when this gets cut down to 4:3 (or 1.33:1 ratio? That's right, we lose 43% of the screen width. There is a scene toward the end of this movie where Dennis Hopper is threatening Kyle MacLachlan with a gun. Well, in this wonderful Pan & Scan edition, we get to see Hopper threaten the left side of the television with the gun while talking menacingly to it, as MacLachlan is nowhere to be seen. Wonderful stuff.
For practical examples of Pan & Scan, you might want to check out IGN's visual comparison of Pan & Scan and Widescreen editions of some famous movies (Star Wars, Ghostbusters, The Usual Suspects) at http://dvd.ign.com/news/16377.html.