central air is when the large AC unit is located outside and it cools the whole house through air ducts
in the SW(dry climates) they also use "swamp coolers" as a cheaper alternative to refridgerated air since it also adds moisture to the air as it cools instead of taking it out like refrigerated air does
if you want me to get into how a swamp cooler works i will, but you could probably google it
i would suggest getting a 6000-8000btu AC unit. if you have a window in the garage you can mount it in that, but if not you will need to buy a wall mount unit and cut a hole in one of the garage walls, preferably on the east wall for maximum effect