I particularly want to know if iPhone 4S can work with other networks that use SIM cards when the device is authorized to be unlocked. With these conditions, can it take a SIM card when traveling?
The iPhone 4S is a world-phone, compatible with Verizon and Sprint's CDMA networks in the US and GSM networks worldwide. It has a micro-SIM slot for connecting to GSM networks. Most phones sold in the US are locked to the carrier you buy it from, you may have luck getting them to unlock it so you can use a different SIM. Unlocked iPhones are (tautologically) unlocked, so you can use any SIM card, but they cannot use the aforementioned CDMA networks.