GSM 900 * 2 = GSM 1800
As you can find out by its name that the difference is bandwidth.
GSM networks operate in a number of different frequency ranges. Most 2G GSM networks operate in the 900 MHz or 1800 MHz bands. Some countries in the Americas use the 850 MHz and 1900 MHz bands because the 900 and 1800 MHz frequency bands were already allocated. Most 3G GSM networks in Europe operate in the 2100 MHz frequency band.