In the U.S.A. an ultralight is defined in Federal aviation regulations
FAR Part 103 as a
single seat powered
flying machine which weighs less than 254 lbs, has a top speed of 55
knots (63 mph),
stalls at 24 knots (28 mph) or less and carries no more than 5 gallons of fuel. Excluded from the empty weight are floats for water landings and
safety devices intended for deployment in an emergency. The weight allowance for an
emergency parachute is 24 lbs. so an ultralight with a
parachute could weigh 278 lbs.
There are strict operating limitations, but no mandatory license or registration. Special 2-seat exemptions are granted to instructors for training purposes only. These training aircraft can weigh 496 lbs and carry 10 gal. of fuel. All single seat ultralights which exceed the above limits and any 2-seater not used solely for instruction must be registered as an amateur built aircraft and must be flown by a licensed pilot. Regulations vary outside the USA, but many nations allow more weight, speed, fuel, and 2-seat operations at the expense of more safety requirements. Some call them microlights.
from the Ultralight FAQ at ultralighthomepage.com.