Tetra is the standard for Terrestrial Trunked Radio defined by the European Telecommunications Standards Institute (ETSI). The standard was specially designed for communication between government agencies, security and rescue services such as police, fire brigade, rescue services, the military.
TETRA has a similar cellular architecture as GSM telephone networks, and therefore radios of several other different manufacturers can communicate with each other in one radio network, similar to mobile phones in GSM networks.