In a simple manner, Data card is a kind of dongle that comes with both (GSM or CDMA) option. They all are depend on Telecom service providers, or a dummy data card device (selling by Huawei etc.) without SIM where the buyer can use it their 3G or 4G SIM with pocket internet or internet tariff etc. to use the data card for the system or related device.
This is mainly useful for the net connectivity or data usage (browsing) at the system. Reliance, MTS and other few telecom service providers sell CDMA data card with their own tariff. The monthly package is based on your requirement, usage and tariff. It is all depend on 1GB data usage to 10GB scheme or even unlimited etc. Some of them offer post paid tariff and some of them are offering pre-paid too.
Nowadays, data card is coming with WiFi option which is really useful for the current trend since most of them using their data card for the System, Tab PC and Smartphone etc. In that way, data card with WiFi option is much better to buy which initially cost around Rs 1500, from any telecom service provider followed by monthly rental. I personally tested and felt that CDMA data card is far better than GSM one. Anyhow, it is all depend on the feasibility. I felt CDMA better due to roaming and using anywhere, whereas, GSM data card gives little trouble while comparing both.
Whatsoever, every data card has microSD card slot which you can use it effectively. By the way, buy the data card based on your requirement. In general, post paid tariff starts from around Rs 600 + Tax.