If i buy an unlocked iPhone, can I put any SIM Card in it?

I want to buy an unlocked iPhone 4. I watched a video and you need to put a SIM card in to use it I think. Can I just buy like a Straight Talk SIM and just put it in the slot, and will it work? I'm not too good at understanding this stuff and it would help if someone told me.

Yes you can put your SIM card in it and it will work if your iPhone is unlocked. I have a iPhone 5 that is factory unlocked and I use a T-Mobile SIM card in it but the phone is for Verizon. It will work as long as the SIM card you buy is the same size as the slot on the iPhone. I had to upgrade my SIM card from T-Mobile.

Yes that's what unlocked means!

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