Tri-Band - A network infrastructure or wireless phone designed to operate in three frequency bands: 800 MHz, 900 MHz and 1800MHz. For most, when you leave North America, your cell phone becomes useless due to incompatibilities between the American cell phone system and the GSM system almost universally adopted elsewhere in the world.
In mobile phones, dual-band and tri-band refer to functionality that allows a cell phone to support either two or three frequency bands out of the four major GSM (TDMA) bands - 850/900/1800/1900 MHz.