CDMA and GSM are Radio technologies and mobile Phone is just an Antenna to receive these Radio Signals. Since both the technologies work on different frequency range and has different modulation shemes therfore the mobile phones for both are different.
Apart from this 3G tehchnology which is combination of both GSM and CDMA (WCDMA) is compatible with both GSM and CDMA. So a 3G mobile can serve for both GSM and CDMA radio signals.
SIM (Subcriber Idendity Module) is consists of Credential for network authorization. Since Both GSM and CDMA technologies has different Authorization functions and parameters hence both SIMs are different. Note: In CDMA technology SIM is often called as RUIM (Removable User Idendity Module).
CDMA uses Different Frequencies than GSM. CDMA also does not require a SIM card.
Now some companies do provide SIM cards for CDMA connections also, such as Reliance communications and TATA Indicomm in India.
CDMA can hold more calls per channel on their towers, while GSM drops alot of calls. That is why people are always complaining about T-Mobile and At&t dropped calls. GSM is used internationally while CDMA is mostly used in the domestic United States. CDMA carriers are Verizon, Alltel, Sprint, Cricket....while GSM carriers here in the U.S. are T-Mobile and At&t.
