PTT is a two-way communication service that works like a "walkie talkie".
A normal cell phone call is full-duplex, meaning both parties can hear each other at the same time. PTT is half-duplex, meaning communication can only travel in one direction at any given moment; only one person can be heard at a time.
To control which person can speak and be heard, PTT requires the person speaking to push a button while talking and then release it when they are done (hence the term Push To Talk.) The listener then presses their button to respond.
PTT makes it easy to have short interactions with one or more people throughout a day with the press of one button, skipping the dialing and answering steps required in a normal phone call.
Most PTT systems allow group calling, meaning one person can speak to everyone in their assigned or current group at once, just by pressing a PTT key.
Modern PTT systems introduced in 2003 and later use VoIP technology to provide PTT service digitally over standard data networks. Older types - such as iDEN - used radio network technology designed primarily around PTT.