Lightroom is a digital darkroom and file management software, however it doesn't offer all the finer editing that Photoshop can offer. Thats why many people use both. For myself I usually start work in Lightroom, however if there are finer details I need to edit I just click "Edit in Photoshop" from LR.
There is an advantage to working via Lightroom -> Photoshop. In Lightroom any adjustments you make do not affect the original file, it is Non-Destructive editing. In Photoshop you need to save your edited file as a duplicate if you want to keep the original intact.
When I click "Edit in Photoshop" inside LR, I have my preferences set to "Edit a copy with Lightroom adjustments" because though Lightroom offers non-destructive editing, other programs do not. Therefore Lightroom allows you to edit a copy of the file in another software, usually as a Tiff file (PSD format is available for PS), and when I finish editing in the supported external program/plugin I just close it and Lightroom re-imports the edited Tiff file for you to continue working.
Other reasons why I use Lightroom for managing my photos:
1) Management of Keywords/tags, metadata, etc
2) Ability to work on massive amounts of files at the same time if needed. e.g. Sync adjustments across multiple files
3) Ability to export multiple files easily, either onto local drives or to online galleries such as Facebook, Flickr, Smugmug.
4) Helps sorting files inside catalogs by date, time, etc.
I think there is a trial version of Lightroom available, try it out. Also Scott Kelby wrote a nice book on Lightroom: The Adobe Photoshop Lightroom 3 Book for Digital Photographers.http://www.adobe.com/products/photoshoplightroom/
For the trial version and some slideshows/videos showing what you can do.
This is a book by Pro photographer David duChemin which I recommend reading after you finish with Scott Kelby's book. Vision and Voice: Refining Your Vision in Adobe Photoshop Lightroom
As for file and disk management, Chris offered great advice.
For my workflow, I never delete a file unless it's a file I don't want, mostly because it didn't come out right or I accidently pressed the shutter button when I wasn't ready.
Since I work off a laptop, I usually keep only the files concerning the current year on the local drive. All previous years get exported onto 4 external drives:
1) Primary USB drive that I carry around with in my laptop bag if I quickly need to access previous files.
Since my laptop has either a USB 3.0 port or eSata port this drive is a fast portable drive like the Lacie Rugged 2.5" drives.
2) 3x backup drives where I make duplicate copies of all my files. Each stored inside a padded soft bag in separate locations.
In Lightroom I create catalogs for every 2 years worth of photos. I have set Lightroom to backup each catalog once a week, and delete old backups on a regular basis. Normally as long as I have the 3 most recent backups of a catalog, I delete any backup which is older.