In the US, there are many amateur/club-level sanctioning bodies. Check this site for links -->
http://www.racebikesite.com/motorcycle-racing-associations.htm and check out their rule books, as each org has diff rules for road racing (what size bikes are allowed for classes, etc). Im sure youll find answers to your questions.
As far as pro motorcycle road racing in the US, AMA Pro Racing is the national sanctioning body for the sport. AMA has 6 classes: Superbike, Supersport, Superstock, Formula Extreme, 250 GP and Pro Thunder. But next year, the Pro Thunder class will be governed by WERA, and 250 GP will move in 2004.
Moto Grand Prix (MotoGP, 250 and 125), World Superbike and World Supersport (600's) in Europe are sanctioned by the Federation Internationale de Motorcyclisme (FIM) in Europe.
Obviously youll find more Euro bikes in the Euro circuits (Aprilia, Ducati, Benelli, etc) than here in the US. Suzuki, Yamaha, Kawasaki and Honda are the predominant players in AMA Pro Racing with full factory support. Ducati isnt really a full factory supported team in the US although there are 2 teams running in Superbike - HMC Ducati and Austin Blue Bayou Ducati. Most AMA privateers run Suzuki machines.