The Dirtbike is a motorbike made for off-roading, and is arguably the second most versatile vehicle in the game, following the ATV due to its great acceleration, turning and small form factor. The Dirtbike currently has very slow speed, great acceleration, and fantastic handling.
As of the February 9th City/Weapons update, the Dirtbike spawns at the Train Station 1, so you can get away from a Passenger Train robbery without having to walk to a nearby vehicle (which was also a Dirtbike if there was no other vehicle).
The Dirtbike might be laggy on some low-end devices.
In a server where there is vehicle lag, the Dirtbike, Dune Buggy, Volt Bike and the Patrol is generally lag free. (Vehicle lag servers are when you enter a vehicle, but it takes a long time before you can drive it, but for the Dirtbike, Dune Buggy, Volt Bike and the Patrol, you can drive it right away.)
All though called a dirt bike, it's look resembles more of a sports bike, rather than a dirt bike.