The YZ is a motocross bike, the trail or woods bike is a WR426.
A cr250 is a motocross bike
By dirt bike do you mean and enduro bike or a motocross bike or some other bike at all? For any bike except motocross bike you definitely need license, and for motocross bike you don't, because you can't ride it on the streets at all... You can only use it in motocross tracks and for that you need to have a special permit...
For off road, KTM. For motocross for 2009, according to the guys at Motocross Action mag, the Kawasaki KX450F.
Become enthusiastic about motocross bikes.
For professional motocross, the biggest bike that is allowed to be raced is a 450 four-stroke. For local race purposes there is a Vet class that allows any size bike to be raced (usually up to 525 since that's the biggest motocross bike that KTM makes).
2006.
a bmx bike
A fast one.
Dont.
KTM all the way, many models to chose from and race ready from the showroom floor
Motocross was invented by edmann wilkison as a game to show off the dirt bike skills