A phone that supports four different radio frequency bands. Different mobile phone networks around the world use different bands. A phone can only be used on a network if it supports the correct band(s).
Quad-band usually refers to a phone using GSM technology. A quad-band GSM phone is one that can be used for basic service almost anywhere in the world.
That's because most countries have at least one GSM network, and most GSM networks worldwide use one of four bands. By supporting all four bands, a quad-band phone is compatible with most GSM networks worldwide.
The four bands include the 850 and 1900 MHz bands - used in the Americas - and 900 / 1800, used in most other parts of the world.
Compared to a tri-band phone supporting the 900 / 1800 / 1900 bands, a quad-band phone adds support for GSM 850 for full coverage in the Americas.
Compared to a tri-band phone supporting the 850 / 1800 / 1900 bands, a quad-band phone adds support for GSM 900 for full coverage in Europe and Asia.