There's a CDMA model of 4S (not sure if available in India), but it is carrier locked (as far as I know it cannot be unlocked unlike GSM).. So a particular CDMA iPhone will only support SIM of that network operator with which is has been locked to. I don't know if any Indian Network offers CDMA locked iPhone with their SIM..
MEID : A mobile equipment identifier (MEID) is a globally unique number identifying a physical piece of CDMA mobile station equipment. The number format is defined by the 3GPP2 report S.R00 but in practical terms it can be seen as an IMEI but with hexadecimal digits.
ESN : Electronic serial numbers (ESNs) were created by the U.S. Federal Communications Commission (FCC) to uniquely identify mobile devices. ESNs are currently mainly used with CDMA phones compared to International Mobile Equipment Identity (IMEI) numbers used by all GSM phones.
I guess its simple to understand. Now difference between ESN & MEID is :
In handsets having ESN, it can be used by only one operator and these type of handset can't be unlocked to use on another operator.
In handset having MEID, mostly tehre is a SIM Card Slot. So, any CDMA operator Sim card can be used in these kind of handset. Even Handsets not having Sim Card Slots can be used with different operators if necessary firmware is supported by the CDMA Operator.
P.S: MEID Sim Card Slot & GSM Sim Card Slots are different.