GSM1900 is the term used to denote GSM operating in the 1900 MHz band. The 1900 MHz band ranges from 1850 - 1910 MHz and from 1930 - 1990 MHz. Mobiles transmit in the lower band and base stations transmit in the upper band.
GSM1900 is primarily used in the Americas. GSM1900 is developed for the American market because the 1800 MHz band (GSM1800) was not available for mobile communications in that part of the world.
GSM1800 is also known under the old name PCS1900, Personal Communications System 1900.