The category is known also as "Part 103 ultraligtht" (based on a chapter in FAR).
Requiremnets for an aircraft to be an "ultralight" in USA:
Empty weight: 155 lb (unpowered) or 254 lb (powered) excluding floats or safety devices
Fuel capacity: max 5 U.S. gallons
Top speed: 63 mph at full throttle at straight level flight
Power off stall speed: 28 mph
Advantages of ultralights in USA
There are several advantages for homebuilders when keeping aircraft in limits of part 103 ultralight category. First, the airplanes dont have to be registred or inspected or have any standarts. Second, the pilot does not have to hold any pilot licence or medical, although it is very encouraged to have a pilot training to fly an aircraft.