WP post thumbnail coding questions

I think I've decided to use the WordPress Post Thumbnail feature to replace TimThumb. So this leads to a series of questions:

* Does WP store the post thumbnail in the database as a blob, or is it just a reference to a file somewhere?
* How can I import a ton of thumbnails (one per post) into WP and assign them to specific posts?
* Can I also specify a caption or description for those post thumbnails?

I guess the second question is the most important of them all, since I need to bring in a whole pile of images formerly stored in a folder path I just randomly decided on when I originally coded it with the old function.