Two types of grounds are used in automotive electric circuits. Direct ground components are grounded through their mounting points. All other components use some sort of ground wire which is attached to the body or chassis of the vehicle. The electrical current runs through the chassis of the vehicle and returns to the battery through the ground (-) cable; if you look, you'll see that the battery ground cable connects between the battery and the body or chassis of the vehicle.
NOTE: It should be noted that a good percentage of electrical problems can be traced to bad grounds.