For an overview of all skills, see List of Vehicles and Siege Engines Proficiencies.
Vehicles and Siege Engines are proficiencies that are a special kind of mix between Skill Proficiency and Tool Proficiency. The proficiency include everything from repair to actual usage, which results in it requiring its own proficiency category.
If you have proficiency with a certain kind of vehicle (land or water), you can add your proficiency bonus to any check you make to control that kind of vehicle in difficult circumstances. And if you have proficiency with siege engines, you can add your proficiency bonus to attack rolls and any other checks made when operating it.
Vehichles Generalization
Vehicles are affected by Proficiency Generalization. Such that learning to operate machinery or heavy transports makes it easier to learn how to operate slight different machinery or transports.