lightroom is a raw converter, which you use to convert your out of the camera raw-files to TIFFs or JPEGs. If you shoot RAW, there are endless ways to adjust the image when you convert to a standard image format (you can still tweak exposure, contrast, white balance, and much more).
As this is similar in a sense to what people do in their labs when they produce prints from the film, Adobe called it lightroom (as opposed to darkroom

).
There are many other RAW conerters around, Apple's Aperture being one, or Bibble, or BreezeBrowser, Phase One, ...
Of course PS2 has the Camera-RAW module these days, but that is sort of not very workflow optimised

. With the other "proper" converters you usually get a very decent RAW-image browser, batch processing,...