Exactly how does SIM cards on iPhone 5 work?

Alright so i want to buy a iPhone 5 in Serbia while i'm there for vacation because it's cheaper there. Thing is i know SIM cards don't work the same as in Sony Ericsson mobiles (or i think it don't)
So if i go to Serbia and buy my iPhone 5 there, and go back to Sweden, how does it work? Will i be able to buy myself a Swedish SIM card and use the iPhone 5 with a Swedish provider? Please i need answers fast!

It would need to be an unlocked iphone 5 to work with a gsm sim card in sweden

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