A garage was an enclosed place used for parking automobiles. In the case of residential garages, they were typically part of the building, although they could be separate structures altogether. They were accessible via the home owner's drive, and usually contained tools for working on such vehicles as well.
Other garages were used as shops for mechanics, or grease monkeys, to repair such vehicles. One such shop, called Chamey's Automotive Repair, was located on Sigma Iotia II. (TOS: "A Piece of the Action")