What's bowl (pipe)
Originally used exclusively for the recepticle of a pipe, the term has come into use to differentiate between pipes used for smoking tobacco and pipes used for smoking marijuana.
Bowls of this kind vary greatly (visually) from one to another. A few reasons being the different 'varieties', materials they're made from, and the fact that most are hand made. The common thread between them are components. All of the pipes have a receptacle of some sort, a "stem" between this and the final part, a "mouthpiece". The smoking material is placed in the receptacle and affected with a heat source while air is drawn through the bowl and stem to the user.
Some bowls have a "carburetor" or "carb" (also referred to as a "shotgun", "choke", or a "rushhole" by users), a small hole usually located near the recepticle to allow fresh air in. Two reasons for this are to clear the pipe of smoke, and to conserve smoking material when enough smoke has been created.
Some examples of bowls. Glass and aluminum bong bowls, glass pipe and metal bowl from a pipe that disassembles. These are examples of bowls for marijuana use.
Metal bowls are generally made from pieces used in lamps or (sometimes) plumbing. They are often able to be disassembled for cleaning, to remove resin for smoking or for easy storage. They often have a 'sleeve' over the stem area to keep heat from burning the user or just for decoration. The sleeves can be made of wood, heat-resistant plastic, clay, decorated metal or made by the user commonly out of electrical tape. In some places these pipes can be 'custom built' as the buyer chooses the pieces for their own unique piece.