Adobe Photoshop Lightroom 4 is the latest version of Adobe's complete, end-to-end, photo management and editing solution for both amateur and professional photographers. With version 4, Adobe Photoshop Lightroom adds a number of new features while at the same time, continues to make itself the premier software product directed at photographers who want to bring the best out in their photographs.
Lightroom is a software based digital darkroom that is used for developing your photographs, managing your photographic library, and creating great presentations whether for printing or for the web. For a complete list of requirements to run Lightroom, check out the Adobe Photoshop Lightroom system requirements page
For those who are not familiar with Adobe Photoshop Lightroom, I find that there is still some confusion between what the difference is between Adobe Photoshop and Lightroom. The fundamental difference is that Photoshop is an image editing program that has big guns to do just about anything to an image. Many times, at least for most traditional photographers, it has too many things. While Photoshop comes with Adobe Bridge and Adobe Camera Raw, they are more challenging to use and really do not provide the same kind of management that you get with Lightroom.
With Lightroom you have everything that is needed to perform photographic processing (it contains the same camera raw engine that comes with Photoshop) but it has the ability for the management of your images that is far easier and, at least to me, more superior than is available with the Adobe Bridge.
Does that mean that you no longer need Photoshop? For some that answer would be yes. In fact, Lightroom is the better choice especially for those who do not do a lot of advanced processing. For others, Lightroom is a better way to get to that point where they send their file to Photoshop for more enhancements. It really comes down to what you do with your images beyond primary processing.