These are the two different means of mobile communication being presently used worldwide. The basic difference lies in the Multiplexing method used in the aerial communication i.e. from Mobile Tower to your mobile and vice versa. CDMA uses Code Division Multiple Access as the name itself indicates, for example you are in a hall occupied with number of people speaking different language.
You will find that the one  language you know will be heard by you and the others will be treated like noise. In the same manner each CDMA mobile communication takes place with a “code” communicating between them and the other end if one is knowing that code then only it can listen to the data being transmitted i.e. the communication is in the coded form.
On the other hand GSM (Global System for Mobile Communications)uses narrowband TDMA, which allows eight simultaneous calls on the same radio frequency. TDMA works by dividing a radio frequency into time slots and then allocating slots to multiple calls. In this way, a single frequency can support multiple, simultaneous data channels.   _{Your Comment}