Adobe Lightroom is (basically) a digital dark room targeted at professional and amateur photographers.
It allows you to improve/alter photographs by adjusting histograms, color balance, exposure, contrast, etc.
It has a good workflow and organizational (as well as tagging) system.
The interface and tools are better organized and somewhat simpler than Photoshop.
Unlike Photoshop, it does not handle compositing, special effects, type, video, etc.
Like I said, “digital darkroom”. Apple’s Aperture is extremely similar and competes with it.
I used Lightroom for a year, then switched to Aperture a couple years ago.