LR3 does three basic things - it catalogs all your images, it applies 'processing', and it creates exported media (prints and web galleries). LR4 adds more features such as GPS tagging. LR can't be directly-compared to PS as it doesn't aim to do the same things, but it can be compared to Bridge:
- The catalog system is a database rather than a file browser (which you get with Bridge), so it can index offline content on external drives, etc.
- Everything LR does to an image in the processing panel is nondestructive, so it doesn't alter your original file - unlike in PS. This means you have much more freedom to experiment, and creating many 'virtual copies' of an image doesn't eat your disk space.
- The print and web export features in LR are way more powerful and flexible than in Bridge.
- PS, Bridge and LR can process raw images, and they use the same Camera Raw engine - so the effects are the same - however LR's noise reduction and toning controls are considered to be much better than the tools in CR or PS.
- Bridge will browse pretty much anything (images of any kind, PDFs, video, PDF, InDesign, etc.) - LR only indexes photos and video.
- Both LR and Bridge can import files from your digital camera, plus LR has the option of tethered shooting for a few models.
- Both LR and Bridge can link to a social media account (Facebook, Flickr, etc.) and upload files directly to your albums, provided you're using the US English version.
- While LR has sa couple of touchup tools (spot removal, etc.) in addition to the developing tools (curves, tone mapping, etc.) it's not in any way a replacement for PS.
- If you work with raw files, LR4 is designed to work much better with DNGs - it's adding things like multi-core preview rendering and embedded thumbnails so viewing a large folder of DNGs is many times faster than in Bridge.
Suggest you watch JK's video here for more : http://tv.adobe.com/watch/the-complete-picture-with-julieanne-kost/should-i-use-lightroom- or-bridge/