Encapsulation and control of visibility count as the most important ideas of object-oriented programming.
A class and its interface to other classes. The interface is often called the client interface. In this illustration the operations Op1, Op2, Op3, and Op4 form the client interface of the class.
The visible parts of a class constitute the interface of the class, as seen by client classes
The interface - or the client interface - make up those parts of the class that can be used from client classes. The non-interface parts of the class are private to the class.
Read more about classes in the text book version of this material.