Ultralights operate under FAR part 103. They do not require a license to operate. I flew them back in the 80s as a fun pass time.
I am a licensed private and instrument pilot. This allows me to legally be able to fly single engined, land based, non turbine airplanes up to 12,000# takeoff weight.
The airplane in my signature, a Kitfox model IV, is a "light sport" eligible aircraft which is a newer category that allows for a more limited license (cheaper). My license covers these requirements also. Light sport is speed limited to 120 knots and 1320# max takeoff weight (among other things).