Amongst the biggest advances in digital photography over the last few years has been the rise of Raw shooting, especially using DSLRs. By using the so-called digital negative, you have far more control over your images and can achieve significantly higher quality than shooting in camera-processed JPEG.
Of course the popularity of Raw means that there's a need for good, easy to use software in order to process the images. There's a number available, from the high end Phase One package to simple and effective Bibble Light. Manufacturers usually bundle in something simple with their cameras too, and of course there's the ubiquitous Adobe PhotoShop in all of its incarnations.
Photoshop relies on the Adobe Camera Raw module for editing and conversion of Raw files, and this tremendous bit of software is incorporated in all of the Photoshop family. It goes without saying then that it lies at the heart of Adobe Photoshop Lightroom.
Lightroom works differently from older applications. Instead of applying edits to the image directly (and destructively), it saves the edits as data in a "sidecar" .xmp file so your original file is never damaged. You can go back at any time to the original file should you need to. Of course the application is also equipped to handle JPEG and TIFF files, as well as Raw files from all major cameras, including DNG.
The second major point about Lightroom is its interface. It is a simple modular application, with five steps allowing you to organise your images in the Library module; edit in the develop module; print in the unsurprisingly named print module; make slideshows in the you've guessed it slideshow pane and finally produce galleries in the web module.
This step by step process forces the user into working within a logical and co-ordinated workflow, without restricting individual working practices.