Technichally an ultralight is not an aircraft. In the US nither the Pilot nor the aircraft has to be licensed or registered. I can't remember the exact weight but I think the Max gross weight is 250lbs to be an ultralight. I think there is an additional couple of pounds allowed for a two seater.
As far as flight instructing is concerened I belive that it is a case of owner-supplied aircraft. If I happen to build say an experimental with a tailwheel I can hire a flight instructor to give lessons to get my tailwheel endorsement in that airplane. A flight instructor can't own an experimental aircraft and then look for students to instruct in it.
I hope this explains the diffenernce. Correctly.
OBAMA-WORST PRESIDENT EVER....Even SKOORB would be better.