Can I use an iPhone with Sprint?

If, for example, someone were to sell me an iPhone from Sprint could I put my Sprint SIM card in and use it? I'm not familiar with Sprint at all so excuse me if this is a silly question. I've use AT&T since I've my first cell phone, but I recently switched to Sprint and am not familiar if this is allowed or able to be done like AT&T phones are able. You don't need to explain the CDMA -- GSM thing. I understand that, but I'm uncertain about the aforementioned issue at hand.

If it's from sprint originally, and you're using sprint now, then yes, it should work.

It seems like you're unaware of the CDMA-GSM thing. Sprint doesn't use SIM card for regular usage. They might use one for LTE devices but they don't use SIM as they're CDMA. But you can just get the phone activated on your Sprint account from the store. Just go to a Sprint account and tell them that you want to activate this iPhone on your current plan.