Adobe Lightroom is a software for photographers which provides an integrated image management database + a post processing application. It is not a replacement for image browsers (such as IrfanView) or graphic editors (such as Adobe Photoshop or GIMP).
Lightroom provides the user with a system to seamlessly manage the entire digital photography process, from importing images to editing and post-processing eventually leading to printing and online publishing.
Lightroom provides the user with a database for organizing and storing the images, presets to perform frequent edits, additional editing tools in the develop module, a facility to prepare a slideshow for presentations, the ability to print images and also publishing on the web.
For further details, check this detailed review of Adobe Lightroom