A communicator is a small transmitter and receiver that sends signals through subspace, allowing real-time communications between an orbiting starship and an away team on a planet's surface. A communicator can also serve as a security token, allowing restricted commands -- such as control of security force fields -- to be issued by voice through the communicator (TNG "The Hunted").
While they have at least enough range to reach a ship in orbit, communicator transmissions do not penetrate dense materials well. If a starship's internal communication system fails, individual communicators are not sufficient to provide ship-wide communication through the ship's bulkheads. A variety of other phenomena, including electromagnetic radiation and various types of particle radiation, can interfere with communicator transmissions.
In the 23rd century, a communicator was a hand-held device normally carried in a case attached to the user's belt. In the 24th century, communicators have been further miniaturized and built into the insignia worn on Starfleet uniforms. 24th-century communicators also serve as transponders, allowing a friendly starship to locate a functioning communicator at will.