An interface is a reference type in programming that defines a contract for classes to implement. It specifies a set of methods, properties, and events that must be included in any class that adopts the interface. Unlike classes, interfaces cannot be instantiated directly; instead, they serve as blueprints for creating classes that adhere to the defined structure. This allows for abstraction and promotes a modular design, enabling different classes to interact seamlessly. Interfaces are particularly useful in languages like Java and C#, where they facilitate multiple inheritance and help maintain a clean separation of concerns in software development.