A head shop is a store which caters to drug users and dealers. They can be found in most major cities, and hawk pipes and bongs of all types. In many head shops in the U.S., it is possible to buy preparations of the legal psychedelic plant Salvia Divinorum. Some head shops may also sell crack-pipes or other accessories related to hard drugs, but most do not. Most also sell scales, and some also sell empty plastic bags sized specifically for the most common amounts marijuana is sold in. A lot of shops offer various preparations designed to allow one to pass a drug test, although most (if not all) of these products are scams. Odor removal sprays are also common. Some places also hook up customers who don't seem like undercover cops with drug dealers, although most head shops do not even allow you to mention illegal drugs, and will not sell you anything if you do. Always say "tobacco" when asking about something. Many shops also sell T-shirts, cigarettes, specialty tobacco, posters, incense, and novelty items.
Head shops exist in a very grey area of the law in the United States. Much drug paraphernalia is banned under federal law. However, head shops do attempt to get around this by insisting that their products are not for drug use. This may not matter however; a bong meant for "tobacco use" is still a bong, and manufacturers and sellers have been arrested in the past. Most head shops exist because they are tolerated by the local authorities. Some stores which cater to counter-cultural types may also have a back room containing drug-related items which requires permission to access, thus making themselves a little less conspicuous.
Head shops usually exist in the "sleazy" area of town. If you're walking down the street, and see a massage parlor, a porn shop, and a tattoo parlor nearby, you may very well be near a head shop. Most places have names which don't sound related to illegal drugs, and may present themselves as "gift" shops. Many shops in suburbs are in unincorporated areas, thus reducing their hassles with police.