The following is from Apple's Swift Programming Language book:
"A protocol defines a blueprint of methods, properties, and other requirements that suit a particular task or piece of functionality. The protocol can then be adopted by a class, structure, or enumeration to provide an actual implementation of those requirements. Any type that satisfies the requirements of a protocol is said to conform to that protocol."
– Apple Inc., The Swift Programming Language (Swift 3.0.1), iBooks
Protocol-oriented programming is a vast topic that also deserves coverage. It is the subject of many discussions, and I won't dive into it in depth. However, let's go over the basic concepts, as they will be useful for understanding some concepts that will be explained later in this book.