Yes, an unlocked iPhone will work overseas. An unlocked iPhone is a phone that isn't tied to a specific carrier. That means that you can use a SIM from any carrier (and any country) in your iPhone, and it will work just fine. When you buy your iPhone straight from Apple or through a third-party vendor, that phone is already unlocked.
With refurbished iPhones, be careful—some are sold locked. That can be a problem if you choose a phone locked by a carrier that isn't yours, or if you are planning to stay overseas for an extended period of time.
Browse unlocked iPhones only to be sure that your phone will be able to work abroad with an international SIM card.