2 frequency bands, used by GSM services in the USA. 2 different frequency bands are used by other GSM services elsewhere in the world.
If you travel a lot then you wabt to be able to us your phone anywhere, you'd want a phone that can work on all four bands - commonly called a quad-band phone. While these are still somewhat rare,they can also be expensive.
You can search for a certain brand name here
 Unlocked Cell Phone Touch Quad Band AT&T T Mobile GSM  US $48.99
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 New Unlocked Cell Phone Touch Quad Band AT&T T Mobile US $48.97
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 New Unlocked Quad band camera china mobile phone EG1 US $5.50
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 Unlocked Cell Phone Touch Quad Band AT&T T Mobile GSM US $48.99
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 Unlocked Cell Phone Touch Quad Band AT&T T Mobile GSM US $48.99
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 New Unlocked Cell Phone Touch Quad Band AT&T T Mobile US $48.97
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 Unlocked Cell Phone Touch Quad Band AT&T T Mobile GSM US $48.99
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 Unlocked Cell Phone Touch Quad Band AT&T T Mobile GSM US $48.99
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 Unlocked Quad-Band Dual SIM TV WIFI CELL PHONE F006 US $129.00
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 New Unlocked Cell Phone Touch Quad Band AT&T T Mobile US $48.97
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