a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior. Data field members (member variables or instance variables) enable a class object to maintain state. Other kinds of members, especially methods, enable a class object's behavior. Class instances are of the type of the associated class.

Every class implements (or realizes) an interface by providing structure (i.e. data and state) and method implementations (i.e. providing code that specifies how methods work). There is a distinction between the definition of an interface and the implementation of that interface. In most languages, this line is usually blurred, because a class declaration both defines and implements an interface. Some languages, however, provide features that help separate interface and implementation. For example, an abstract class can define an interface without providing implementation.

Languages that support class inheritance also allow classes to inherit interfaces from the classes that they are derived from. In languages that support access specifiers, the interface of a class is considered to be the set of public members of the class, including both methods and attributes (via implicit getter and setter methods); any private members or internal data structures are not intended to be depended on by client code and thus are not part of the interface.

The object-oriented programming methodology is designed in such a way that the operations of any interface of a class are usually chosen to be independent of each other. It results in a layered design where clients of an interface use the methods declared in the interface. An interface places no requirements for clients to invoke the operations of one interface in any particular order. This approach has the benefit that client code can assume that the operations of an interface are available for use whenever the client holds a valid reference to the object.

The buttons on the front of your television set, for example, are the interface between you and the electrical wiring on the other side of its plastic casing.