The guiding theory behind CSS frameworks is saving time. If you write a lot of CSS, you know you write a lot of the exact same code over and over and over. There is something to be said of learning through repetition, but seriously, how many times do you need to type margin: 0 before it's just annoying?
CSS frameworks are usually just a collection of CSS files that set up things like basic typography, form styling, grids/simple layout, and resets.
Why use a framework? Besides saving time, there are lots of reasons: standardizing the organizational structure of your sites, avoiding dumb little mistakes, and browser compatibility. Sounds pretty compelling, but there are certainly reasons not to use them as well. For example, a framework might be too much for the site you are designing and it's basically just unnecessary bloat. Another common argument against frameworks is that CSS and XHTML go hand in hand, they need to be designed and constructed together, so using pre-written CSS just isn't an option.
As for me, I'm on the fence. I'm actually kind of a "tweener". I have a little base folder that has an index.html file, a style.css file, and an images folder. The index file has a super simple structure in it, the CSS file has some very basic styling in it, and the images folder is empty. I'm pretty happy with this system, but I'm thinking about growing it into a more fleshed out framework.
Hey, this is supposed to be a Roundup! Oh, right.
Every one of these has a different approach. I'll let you explore for yourselves.
Wow, there sure are a lot of them aren't there? I may work on putting one together myself, but I'll need to think of some kind of clever twist to make it more than just another brick in the wall.