Dirt bikes are just like motocycles. But, they arn't supose to be on the road unless you have a dual dirt bike. Dirt bikes are for trail riding and for racing. If you race a dirt bike it is called either Motocross (MX) or Supercross (SX). There are many companys that build and sell dirt bikes like Honda, Kawasaki, Yamaha, Gas Gas and more. Dirt bikes arn't safe without the gear. Which included Jersey, Pants, Boots, Helmet, Googles, Chest Protecter, and some times more. If you want more info on dirtbikes you can always look on line like google.com and look under MX, Motocross or dirt bikes. There are also magazines at any store that sells magazines.
Most of what she said is true. Dirt bikes are motorcycles, true, but almost have more in common to a mountain bike than a Harley. They have lots of suspension travel, and have large wheels. Big bikes are considered anything above 125cc motors, with the most common big bikes now being 250cc four strokes and 450cc four strokes. They are meant for racing, single track riding, desert riding, sand dune riding, motocross, arenacross, enduros, hare scrambles, cross country races.. etc. The "Big Four" manufactures are Honda, Kawasaki, Yamaha, and Suzuki, however KTM (an Austrian brand) is up there as well. Proper gear is necessary, and for motocross consists of boots, pants, jersey, chest protector, gloves, neck brace, helmet, goggles.