The problem is each wireless system has to have its bandwidth that needs to be shared among several users. That is the reason that multiple access techniques have been designed.
FDMA - Frequency Division Multiple Access
TDMA - Time Division Multiple Access
CDMA - Code Division Multiple Access
FDMA - Each user has its part of overall frequency band i.e. the whole bandwidth is divided for many users.
TDMA - The whole bandwidth is reserved for particular user for only specified period of time.
CDMA - User can use whole bandwidth for all time but has to use different code.
As for explanation of difference between these three multiple access technologies there is an example of room with people talking in pairs.
When they would use FDMA the room (whole bandwidth) should be divided into smaller rooms and each room would represent one frequency. Walls would be the separation of these frequencies hence people in pairs would have a chance to talk at the same time without disturbing other pairs.
As for TDMA each pair in the room would have some specific time to talk when other people should remain in silence.
CDMA would allow to talk for all pairs but each of them would have to use different language (code).
This example as described in "UMTS". Javier Sanchez, Mamadou Thioune ISBN: 978-1-905209-71-2 January 2007, Wiley-ISTE
To increase system capacity it is common to use these technologies together. GSM would use for example TDMA together with FDMA.