I get this question all the time from newcomers, so I'll answer it here as we build a proper FAQ:
Q: What's the difference between a RC plane and a UAV? And why doesn't the AMA allow UAVs on its fields?
A: The generally-agreed distinction is that UAVs have the capacity for autonomous flight and navigation. A RC plane becomes a UAV with the addition of an autopilot.
The AMA doesn't allow UAVs because its insurance doesn't cover them. I actually agree with the policy, since our UAVs are by nature experimental and sometimes out of control. We're pretty careful to test away from people.