I've heard about Adobe Lightroom before, but never really understood what's so great about it, or why you'd pay for it when you can get Picasa for free. 

I know Lightroom probably has more sophisticated editing options, but what are they, exactly, and how to do they compare to Picasa? Why would you use one over the other? Is Lightroom better for streamlining editing workflow, but Picasa is better for organizing & tagging photos? And would you ever use *both* programs?

Basically, what are the differences between Picasa and Lightroom?