In India we have more companies providing GSM service like Airtel, Reliance, Vodafone, Videocon, Aircel, etc.
CDMA stands for 'Code Division Multiple Access. CDMA, a proprietary standard designed by Qualcomm in the United States, has been the dominant network standard for North America and parts of Asia.
In India companies providing CDMA service are Reliance, Tata Virgin, Tata Indicom.
Prepaid means, first you pay then you use the service and postpaid means first you use then you pay. In pripaid you buy top up vouchers and in postpaid you pay bills.
In post paid sim is a billing where you pay the bills at the end of the month according to your usage. In prepaid sim you pay before you use the service, you use according to the balance you have.
In CDMA you can't use any other sim in the CDMA phone, and in GSM you can put any sim and the device works.
The difference is
Post paid - you pay the bill later.
pre paid - you recharge first and then use.
Gsm -sim which is compatible with any handsets other than cdma.
cdma - sim that is compatible with only cdma handsets.
The main differences are
Post-paid - To take a postpaid connection your address need to be verified and some providers required security deposite also.You need to pay the bill after use(monthly basis).
Pre-paid - Prepaid connection policies are simple compared to postpaid connection.You need to recharge the sim (your number)before use.And you can use your credits after recharge.
GSM - The gsm SIM you can use with any compatible handset that support GSM sim. Compairing to CDMA simcard it is more user friendly who are changing the simcard (Or handsets) frequently.
CDMA - CDMA simcards are compatible with only cdma handsets.These handsets using CDMA technology.