Do I need a license to listen in on broadcasts, or only to broadcast?
You just need a license to transmit. You can listen all you want.
How often are unlicensed broadcasters caught/prosecuted?
It depends on what you are doing. If you are causing trouble, local hams will try to find you pretty quickly. If you are operating from a fixed location, they probably will. If you steal some obscure person's call sign, and play it cool, you may never get caught.
But illegal operators do get caught. Many hams out their would love the chance to try to track someone down. You should avoid operating illegally from a fixed location, especially if that location is your house. Also, transmit only for brief periods of time. Transmitting continuously helps them find you.
What all could I be charged with, if caught?
You would be fined, possibly several thousand dollars.
Read
FCC Enforcement News.