The system of mobile telephony is based on the principle of subdividing the territory - that may be covered by more network operators - in elementary service areas, called cells, each served by a base radio station. The radio mobile terminal that allows users to connect to the network and make their calls, is made of a main terminal (the so called mobile telephone) and a microprocessor electronic module, called SIM card (subscriber identity module). When you call a user who is out of the area covered by your operator, that user is roaming with another operator.

In a mobile telecommunication network, also called cellular network, signals are irradiated by several fixed antennas located on the territory, one for each cell (here comes the name). Every antenna transmits on a different channel in order to avoid interferences.

(source: Treccani.it)

see also Cellular Telephony in the Learn & Teach section