GSM (Global System for Mobile Communications) use TDMA (Time-Division Multiple Access) technology. In this technology the frequency band is divided into multiple channels and all channels then stacked together into a single stream. In this technology same channel at the same time allows several callers.
In CDMA (Code Division Multiple Access) after digitized it spreads data out over the channel . To keep the signal distinct entire channel assigned its own “sequence code”. Since it allows greater frequency reuse CDMA offers more efficient use of an analog transmission. CDMA was actually invented for the military in field communications during World War II because it has greater security than GSM/TDMA and it has been traditionally faster than GSM