Cascading Style Sheets (CSS) are critical to web layout these days. If you do any web work at all, you owe it to yourself to have at least a passing knowledge of CSS, while most of us need a solid grasp of the subject.
Just because CSS is nearly ubiquitous, however, doesn't mean that it's easy. In fact, CSS can be just about enough to drive you batty when it's not working the way you expect. In those moments, in can be helpful to go back to basics and make sure you're not missing something important.
Whether you're learning or refreshing, Smashing Magazine has an article that just might help you out: Mastering CSS Coding: Getting Started. This certainly isn't a comprehensive reference, but it's a great high-level overview.