Vehicles are mainly cars that player may find in the streets of Tony Hawk's games levels.
Vehicles were in THPS games since the very beginning. Skater could interact with car in 2 ways. If skater will touch any vehicle on the ground, it will make them bail. Taxi cab drivers were yelling at skater in this case. If skater ollied on the car, he is bumped up with Boneless animation, performing a trick called Car Plant.
THPS3 generally follows these rules, except now vehicles are able to brake if skater is on the way. If skater touches a vehicle that is staying in one place, he will not bail.
Since THPS4 vehicle interactions were drastically changed. Now car can't bail skater in any way. Even more, skater can stay on the car now and travel around the level. In addition to this, Skitchin move was introduced, allowing to link to the vehicle and gain additional speed.
THUG added the final improvement, allowing player to ride a vehicle in the game. This feature was controversial though.