Here's how the HPRA defines the vehicles:

STOCK - Stock vehicles are vehicles with any number of wheels. Vehicle may not incorporate any form of fairing, or part of fairing. Wheel discs/covers are allowed.

MULTI-RIDER - Same rules as STOCK, except must have two or more riders. 

These represent "STOCK" vehicles:

 SUPER-STOCK- vehicles are vehicles with any number of wheels. Entries may have any number of riders, male or female. Wheel discs/covers are allowed. A front or a rear fairing is allowed. The edge of the fairing toward the rider may not penetrate the imaginary plane formed by joining the rider's shoulder centerpoints with hip centerpoints. No part of the fairing may be moved in entering, exiting, starting or stopping. 

These are "SUPER-STOCK" vehicles:

SUPER-STREET- vehicles are vehicles with any number of wheels. Entries may have any number of riders, male or female. Wheel discs/covers are allowed. Fairings can be one of two formats.
Type 1: Any amount of fairing is allowed, but no part of the fairing may be moved to enter, exit, start, or stop the vehicle.
Type 2: Any amount of fairing is allowed, but no more than 40% of the fairing surface may be of a hard material (i.e. composite, metal, or molded plastic), while the remaining 60% of surface area of the fairing must be of flexible, woven material (i.e. nylon, spandex). These are also known as SOCK CLASS vehicles. Parts of the fairing can be moved to facilitate entry and exit, or stop and start of the vehicle. The rider's head must be exposed above the fairing. 

These are "SUPER-STREET" vehicles:

STREAMLINER and UNLIMITED - STREAMLINER vehicles are vehicles with any number of wheels and any amount of fairing allowed. Entries may have any number of riders, male or female. Fairing panels may be moved for entry, exit, starting, or stopping. Vehicles do not need to be capable of self-start/self-stop. Vehicles without start/stop capabilities will start at the back of the pack. Rider(s) must be able to exit the vehicle unaided in case of an emergency.