Ultralight aircraft in the United States are much smaller and lighter than ultralight aircraft in all other countries.
In the USA ultralights are classified as vehicles and not aircraft and are thus not required to be registered nor is the pilot required to have a pilot license or certificate.
Ultralight aircraft (United States) Wikipedia
Regulation of ultralight aircraft in the United States is covered by the Code of Federal Regulations Title 14 (Federal Aviation Regulations) Part 103 or 14 CFR Part 103, which defines an "ultralight" as a vehicle that:has only one seat
Is used only for recreational or sport flying
Does not have a U.S. or foreign airworthiness certificate
If unpowered, weighs less than 155 pounds
Ultralight vehicles and their component parts and equipment are not required to meet the airworthiness certification standards specified for aircraft or to have certificates of airworthiness.
Operators of ultralight vehicles are not required to meet any aeronautical knowledge, age, or experience requirements or to have airman or medical certificates.
Ultralight vehicles are not required to be registered or to have registration markings.
Ultralight vehicle cannot be flown except between the hours of sunrise and sunset.
Ultralight vehicles may be operated during the twilight periods 30 minutes before official sunrise and 30 minutes after official sunset or, in Alaska, during the period of civil twilight as defined in the Air Almanac, if:
- Weighs less than 254 pounds (115 kg) empty weight, excluding floats and safety devices
- Has a maximum fuel capacity of 5 U.S. gallons (19 L);
- Has a top speed of 55 knots (102 km/h; 63 mph) calibrated airspeed at full power in level flight
- Has a power-off stall speed of 24 knots (45 km/h; 28 mph) calibrated airspeed or less
- The vehicle has an operating anti-collision light visible for at least 3 statute miles
- The flight is only in uncontrolled airspace
Ultralight vehicles cannot be flown over any congested area of a city, town, or settlement, or over any open air assembly of persons.
Weight allowances can be made for amphibious landing gear, and ballistic parachute systems.
In the United States, while no license or training is required by law for ultralights, training is highly advisable.