With the imminent release of the iPhone 5s and iPhone 5c, plenty of people are considering trading in their current model to help subsidize the cost of their new phone. If you aren't sure exactly which model you have, never fear. Apple has a handy cheat sheet online that makes it easy to sort out.
Apple shows how to identify every iPhone model, including the iPhone 4S which has been replaced by the iPhone 4s
Apple's Support page details every iPhone model ever made, complete with the year it was introduced, storage capacities, where to find the SIM tray, phone images, and more. It's a handy reference guide, and knowing exactly which iPhone model you have when you want to trade up makes it much easier to figure out just how much it's worth.