If you're a newbie, or you just live in smaller production market where there are few, if any, pro audio rental houses, it can be next to impossible to audition different microphones first hand.
This can be limiting, as one of the most abstract concepts to wrap one's head around is microphone pickup patterns, or directionality, which is defined here at mediacollege.com. You either need access to the equipment (like in an academic program), or you just learn on-the-fly, which can be frustrating and inefficient.
The fine folks at Shure have put together a fantastic demo that allows you to audition different mics in a simulated environment. You can choose different mic pickup patterns, distances, acoustic treatments, and background noise levels. Obviously, it's set up to demo only Shure mics, but it's still very useful. (You'll have better results with headphones rather than speakers.)
Link to Shure Online Mic Demo.