The Post Exchange (almost always abbreviated PX) is a store on Army bases which is roughly equivalent to a convenience store in the civilian world. Prices are roughly comparable with a Kmart or Target, but the selection is very limited, often because of space limitations; a PX can range from the size of a walk-in closet to something approaching the size of a Kmart. At the very least, a PX will carry cigarettes, sodas, chew, some snacks, stroke books, male and female hygiene products, and boot polish. The PX system is run by the Armed Forces through a joint service organization called the Army and Air Force Exchange Service (AAFES) but mostly hires civilians to do the work; it is fairly common to see dependent wives/husbands/children working at the PX, especially in foreign countries, because it can be damn near impossible to get a job in the local economy. Two-thirds of the proceeds from AAFES sales are on Morale/Welfare/Recreation services* for military bases ($261.6 million in 2009) but some money goes to improve existing stores or build new ones.
In addition to the conventional PX stores, AAFES also operates fast food franchises such as Burger King, Cinnabon and Popeye's; movie theaters, vending machines, and specialty stores. They do not operate Class VI stores, although beer and wine can usually be found at the PX; they also do not operate the clothing issue stores, although they do carry uniform and insignia items. AAFES has recently expanded onto the internet for the convenience of soldiers and airmen who don't live within convenient driving distance of a PX, i.e. most members of the National Guard and Reserve. The Air Force version of the PX is called the Base Exchange (BX) but is otherwise exactly the same thing.
*This includes everything from intramural athletics programs to swimming pools to libraries to arts and crafts, and quite a few other things that fall under the broad category of MWR.