Adobe Photoshop Lightroom is one of several professional level photo editing and management tools. It is commonly compared to Apple Aperture.
Unlike Adobe Photoshop, which offers a huge set of image editing features applicable to a wide range of fields (including photography but also graphic design, game development, etc) Lightroom was purpose-built for photography. As such it offers features and uses a vocabulary that are much better suited to a typical photographer's workflow.
For version-specific questions use the tags lightroom-3 and lightroom-4
FAQ:
