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.
To control which person can speak and be heard, PTT requires the person speaking to press a button while talking and then release it when they are done. The listener then presses their button to respond. This way the system knows which direction the signal should be traveling in.
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.
New PTT systems introduced in 2003 and 2004 use VoIP technology to provide PTT service digitally over 3G data networks. See VoIP and PoC.