What is an amateur UAV?
An Unmanned Aerial Vehicle (UAV) is an aircraft that has the capability of autonomous flight, without a pilot in control. Amateur UAVs are non-military and non-commercial. They typically fly under “recreational” exceptions to FAA regulations on UAVs, so long as the pilots/programmers keep them within tight limits on altitude and distance. Usually the UAV is controlled manually by Radio Control (RC) at take-off and landing, and switched into GPS-guided autonomous mode only at a safe altitude. (Confused by all the acronyms and unfamiliar terms in UAVs? A glossary is here.)
---1) An RC plane, muticopter (quadcopter/hexacopter/tricopter, etc) or helicopter. You can buy them ready to fly, including autopilot, here. If you want to build your own, these instructions are a good starting point.
---2) An autopilot, such as Pixhawk (see below)
---3) Optional: a useful “payload”, such as a digital camera or video transmission equipment
The DIY Drones community has created the world's first "universal autopilots", ArduPilot Mega (APM) and its next-generation big brother, Pixhawk. They combines sophisticated IMU-based autopilot electronics with free autopilot software that can turn any RC vehicle into a fully-autonomous UAV.
A full setup consists of:
Last but not least is flight safety. The RCAPA guidelines are an excellent set of checklists and do's and don'ts, so please refer to them.
Also, here's the FAA's official word on what's legal and what's not.