Post processing is what is done to a digital photo in an application like Photoshop, after the picture has been taken. If it's too dark, crooked, has funky colors because of the light, has problems because of the lens used and so on, that's corrected in post processing.
I've been watching videos on Kelby Training on how to use Photoshop CS5 and Lightroom 3 to post process photos. I can do many of the same things in Lightroom and Photoshop Camera raw. Lightroom is designed to manage photos in groups better and it is designed to be nondestructive to originals. I would have to work very hard to make permanent edits to the original file using Lightroom. CS5 does a whole lot of things beyond what Lightroom does.
One of the things that I've learned is that most people do the same things to post process a photo. RAW photos have more options than .jpgs. Even if I'm using Camera RAW to post process a .jpg, I won't have those options. I watched one instructor go through his process in Photoshop, then watched a different one go through it in Lightroom. Their explanations for why were slightly different, but only in the word choice. The base reason was the same. The Lightroom class was designed to go into much greater detail.
I had been a bit concerned that I'd have a huge learning curve for Lightroom 3. Since I've discovered the similarities between CS5 and LR3, I'm not concerned. I'll be functional in no time flat.