On a telephone, the handset is a device that a user holds to the ear to hear the audio sound through the receiver. Since the 1920s, handsets usually also contain the phone's transmitter (microphone) which is positioned close to the mouth. In earlier telephones the transmitter was mounted directly on the telephone itself, which often was attached to a wall at a convenient height.
A handset with transmitter and receiver in one unit is also called a transceiver.
A cordless telephone uses a radio transceiver as its handset, and a radio transceiver, wired to the telephone line, as a base station. In a mobile telephone, the entire unit is usually a radio transceiver that communicates through an outdoor base station located at a cell site. Some mobile telephones that can be carried in cars, trucks, and buses look exactly like household telephones, except that their bases are usually screwed or bolted to the interior of the vehicle.