A question that needs a mite more space than we have here to be answered properly, but this is the general jist - a contract iPhone will see you pay less up front, but you'll have to pay essentially a loan on your new iPhone. A SIM-free option will see you pay upfront for the iPhone, but nothing every month for the device after that. This is sometimes known as a 'PAYG' or 'pay as you go' iPhone as well. Most of the deals here are contract iPhones.