Dirt bikes are motorcycles designed for off-road use. These include specifically off-road only bikes, plated dirt bikes, "dual sport" bikes, and "adventure touring" or "adv" bikes.
"Dirt bike" - A purpose built off-road only motorcycle.
"Dual sport bike" - A motorcycle with off-road abilities that is also street legal. Adventure touring bikes falls into this category, being simply larger more street oriented motorcycles.
We are also working on acceptable means of describing the difficulty of a trail. This is being discussed as it pertains to adventure riders here: 
Tags relevant to dirt bike trails:
- dirtbike:scale=[0-6] (See key:dirtbike:scale)
- date_on MM-DD / date_off MM-DD - To indicate trail opening closings.
Other tags/conditions that are pending adoption by the "ADV" community are being discussed here: Talk:Dirt Bike