Cell phones aka mobile phones use radio waves to transmit voice and data. These radio waves are transmitted at different frequencies. They work just like regular radio stations transmission do which is at varying frequencies.
GSM cell phones work within four frequencies 850 MHz 900 MHz 1800 MHz 1900 MHz. Although these frequencies seem very close a phone that works in one of these frequency bands will not work on another band unless that second band is present in the phone. For example if you have a phone that has 900/1800 it will only work on networks that uses these two frequencies. It will not work on any network that uses 850/1900 frequencies. By network I mean the physical medium service providers (at&t and t-mobile in the US) use to move voice and data that is transmitted by their customers.
Example to explain frequencies bands: Let’s equate frequency bands with language. In general they work in a similar fashion.
850 = English
900 = German
1800 = French
1900 = Spanish
The cell phone would equal a person. If the person speaks all four languages he/she can communicate with anyone that speaks any of these languages. If he/she only speaks three languages he/she will be able to communicate with three others who speak his/her language so on and so forth…