Can an unlocked iPhone 4s from Amazon be used with Boost?

I want to buy an unlocked 32 GB iPhone 4s from Amazon because they are cheap. But I'm wondering if I could use it with Boost? I've seen some people say you can't because it's GSM. I don't know what GSM is, so please explain, and would this be legal?

No, You can't since Boost Mobile devices do not take GSM sim cards. If you want an iPhone from Boost Mobile you would have to buy it from Boost Mobile online or a authorized retailer