GSM is the most popular standard for mobile phone service in the world. In more then 200 countries in the world people using GSM mobile service. However, different countries use different GSM Radio frequency for their mobile networks. Worldwide four frequencies are widely used for GSM mobile services. They are 850MHz, 900 MHz, 1800MHz and 1900MHz.
Although 850, 900 and 1800, 1900 frequency are very close, normally mobile phones can't receive more then one frequency at-a-time, a phone that works with one band, can not receive signals of other bands. So if you want your mobile to work with multiple bands, you need to have multi band mobiles. Based on your need, you can go for dual, tri or quad band mobile set.
As a whole, both high and low frequency bands have some advantage and disadvantages. For high frequency service, the network is less effected by the influences of the weather. And you can have more clear and un-interrupted connection. Another advantage of a high frequency is that you only need small antenna. Main disadvantage of higher frequency band is its lower reach. These bands are good for cities where density of mobile users is more. Where as, lower frequency bands are good for rural areas because of their large area reach.
850 MHz is the lowest frequency used for GSM mobile service. As a result of this, 850 MHz network has a bigger and more penetrating reach then the 900, 1800 and 1900 MHz network. Apart from that, the difference between the 850 MHz and the 900 MHz network hardly noticeable. Lower frequency services are good in big building because of their more penetrating reach. A disadvantage of a low frequency network is its low capacity.
900 MHz networks are quite old networks. Like 850 MHz, 900MHz also has a bigger and more penetrating reach then the 1800 and 1900 MHz network.
1800 MHz is a relatively high frequency for cellular phones. Because of this, 1800 MHz networks have a smaller and less penetrating reach compare to 900 and 850 MHz networks. By this the reach in big buildings is low. An advantage of 1800 MHz networks is that the capacity is bigger. This band is idle for cities where density of mobile users high.
1900 MHz available only in North America, some networks in Canada and South Africa. This band is not available in Europe or Asia. 1900 MHz quite similar to 1800MHz. Because of high frequency 1900 MHz band network isn't interrupted by the influences of the weather and you can have a crystal clear connection most of the times. For this network, GSM provider needs to use more mini antennas. An advantage of 1900 MHz networks is that the capacity is bigger.