A desoldering iron is a Soldering Iron made specially for desoldering. Generally they have a hollow tip iron combined with a Desoldering Tool of some variety. I've yet to use one that was terribly effective without modification. A vaccum-cleaner mod to one of these tools is a great budget tool. You can think of a desoldering iron as a middle ground between a regular iron combined with a desoldering tool and a Desoldering Station.

Here is a pic of the $7 radioshack one. It works great for me (Ben Ogle). A cheap mod that can be done to this tool to increase its effectiveness is to cut the red bulb in half and duct-tape it securely to the suction end of a vacuum cleaner. Works much better than having to compress the bulb prior to using it - I made a mess a lot doing that. (Dave B?)