Adobe Photoshop Lightroom is a photography software program developed by Adobe Systems for Mac OS X and Microsoft Windows, designed to assist users in managing large quantities of digital images and doing post production work. Lightroom combines photo management and editing in one interface.
It is not a file browser like Adobe Bridge, but rather an image management application database which helps in viewing, editing, and managing digital photos. Lightroom's closest competitor is Apple's Aperture program which similarly provides photo management and non-destructive editing capabilities.
Read more about Adobe Photoshop Lightroom: History, Features, Market Share