CSS3 is a newer version of CSS with some handy new features like rounded corners, gradients and a lot more. It is still in development, though, and has a long way to go. Some browsers have started to implement it, so you can, for example, use rounded corners in the latest version of each main browser (even IE), but of course the effect won't happen in older browsers.
Beyond that, though, your question is too general. There is a ton of information online, including whole sites dedicated to the topic, so do some reading, and then post some more specific questions if you have any.