Cellular systems use technologies such as GSM, CDMA, TDMA, and AMPS to transmit and recieve phone calls. Each technology uses different techniques to make the most use of the small frequency band given to cellular phone companies. Code Division Multiple Access (CDMA) uses coding for its multiplexing technique. Read more here:
CDMA Explained
GSM stands for Global System for Mobile Communications. GSM defines the entire cellular system, not just the air interface (TDMA, CDMA, etc. Read more:
GSM Technology