Tri-band refers to a handset that can work on all three GSM frequencies. These are:

GSM 900 (BT Cellnet, Vodafone and most networks around the world)

GSM 1800 (Orange, T-Mobile and some of the European networks)

GSM 1900 (Used in USA and Canada)

A SIM Free tri-band phone will work on any GSM network in the world. A tri-band handset that is connected to a line with roaming, will work on all countries that have roaming agreements with that network.

mPhone Motorola