Steve, a regular power mitre saw will only do single anglular cuts. A compound mitre will do double angle cuts, cuts used on Crown Molding. Single angle cuts are great for example picture framing and such.
A compound mitre saw will do a regular angle and a slanted angle on top of that.
Its hard to explain, but check it out. It's only about $30 more.
You cannot cut crown with regular mitre box power saw.