Phones Programs/Partners - Cell phones overseas-best service?

My son will be traveling in India for several months, and I would like to stay in touch with him. Can anyone suggest the best service(s), both as to quality/reliability, and price? I've heard that a "GSM" phone will provide the best service, but is that more expensive? Also, I have also heard that one can purchase a "SIM card" to install in a cell phone - what is it, and how does it work?

As you can see, I don't know very much, so I will greatly appreciate any advice someone can give me.