It's not that CSS and CSS3 are entirely different. CSS3 brings in some new features in addition to existing features of older CSS versions. However, keep in mind that not all browsers fully supports CSS3 at the moment. If you are developing a web application for a target audience that may be using older browsers like IE 6/7/8 you must check that the new CSS3 features works for them or not, so that you don't lose users. So, learn CSS3 but make sure the CSS3 feature you are using works in browser of your target audience. If not, make sure it degrades gracefully for users with old browsers.