I had a doubt whether a apple iphone 4 sprint will work in india or not?

I was buying a second hand iphone 4 from my frnd in US but he told me that it is sprint what does this mean? And will this iphone work in india? If no then how to make this phone work in india? Is this the same as unlocking an iphone.
what will be the charges? What kind of US iphone will work n india? Is it at&t, verizon, sprint. Or something else! Please answer my question in points. The one who answers will get 10 points!

Phones from AT&T and T-Mobile are GSM and will work in India

I think unlocking will work. I'm sure about Verizon, AT&T, and T-Mobile.