A protocol is a set of rules that defines how two objects interact with each other. It defines the methods, properties, and other requirements that must be implemented by any class that conforms to it. For example, the UITableViewDataSource protocol defines the methods that a class must implement to be used as a data source for a UITableView.
An interface, on the other hand, is a collection of related functions and variables that are declared, but not defined. It allows a class to specify its external behavior without providing an implementation for the behavior. For example, the UITableViewDelegate protocol declares the methods that a class must implement to be used as a delegate for a UITableView.