In a nutshell: GSM = 2G, WCDMA = 3G. GSM uses less power and has limited data speed (~144kbps)
WCDMA uses more power and has good data speed (~2+Mbps).
For auto-switching between 2G/3G modes, use the “GSM/WCDMA auto” mode (system balances speed vs battery use based on current signal strength). For 2G-only battery saving mode, use “GSM only”. For higher data speeds (regardless of battery use), choose “CDMA only”.
WCDMA (Wide Band Code Division Multiple Access) or UMTS (Universal Mobile Telephone System) is a third-generation (3G) mobile wireless technology – this standard has been developed to accept higher rates up to 2 Mbps (local area access) or 384 Kbps (wide area access). To obtain higher speeds, HSDPA (high speed downlink packet access) was introduced in version 5 of the WCDMA (3GPP). HSDPA allows data rates up to 10 Mbps (20 Mbps with MIMO).