There are the "ultralights," which look pretty small and "lightweight." Needless to say, these are "VFR only" aircraft. Not sure

The FAA created a new category called "Light Sport Aircraft" (LSA), which has aircraft that's a step up from ultralights. This is also a VFR only license requirement, and it may be "day only." I think the only medical requirement is that you maintain a valid driver's license. LSA pilot training is scaled down from regular private pilot training. It's meant to get more people into the air (and hope they continue their pilot training to more advanced licenses).

Then you have the "regular" private pilot's license which has a medical requirement. It's not difficult, but you have to visit a doctor every 3(?) years to keep your certification. I believe you can fly VFR day or night.

Next up is instrument certification. That will allow you to fly IFR. (Same medical requirement as a private pilot's license.)

Next is a commercial certification. Required to fly for pay, and you have to do a physical every year.

The top of the line is Air Transport Pilot (ATP). That's what is required for airline pilots.

Since this gentleman built his airplane, it's probably considered an "Experimental Aircraft." The performance capability of the RVs probably require a regular private pilot's license.