A mast for cellular networks is a tower upon which antennas are housed for transmitting cellular signals, but the mast transmits no signals itself. Masts can be various sizes, and their size is determined by location, local regulations and the number of antennas attached to the mast.
Many people confuse masts for base stations, but a base station is part of the antennae relay that is attached to a mast. The goal when building masts is to make them as unobtrusive to the local area as possible. Masts may be free-standing, mounted on a structure or camouflaged to blend into their location.