A cell tower is another name for a cell site, or cellular site. It is sometimes called a cell tower because most such sites are elevated whenever possible. Basically, it is a location where a cellular communications system base station (or, more precisely, the base station's antenna) is located. Cell towers typically contain several antennas to cover various directions.
Cell towers can be found wherever an enterprising cellular company has managed to find an elevated location with good sightlines to rent space from. One popular tactic in the United States, as cell towers went up in suburban and urban areas, was to rent space in the steeples of local churches which no longer maintained actual bells, as they usually had good sight lines in at least four directions, were relatively secure, and contained adequate power.