A headset is a headphone combined with a microphone.
Headsets provide the equivalent functionality of a telephone handset with hands-free operation. Like a telephone, headsets typically have only one speaker, but also come with speakers for both ears. Headsets allow comfortable, simultaneous conversation and typing.
Headsets come with a single (monaural) or double earpiece. Double earpiece headsets come in both stereo (two channels of audio signal, one for each earpiece) or binaural (the same audio channel for both ear-pieces). Choosing a monaural headset free's up one ear, allowing you to be more conscious of your surroundings. Telephones offer only single-channel input and output, so double-earpiece designs are only monaural or binaural.
Stereo headsets are standard for computer or other audio applications, where the source offers two-channel output. Telephone headsets generally use 150ohm loudspeakers with a narrower frequency range. This reduces the background noise as sound outside the voice band is less audible. In comparison, Stereo computer headsets use 32ohm loudspeakers with a much broader frequency range and are better suited for listening to music.