A system of cooperative and collaborating objects . An object encapsulates both data and functions. Objects which share the same behavior are said to belong to the same class.
All objects are instances of some class .
Aid to conceptualization and abstraction. One object accesses another object by sending it a message . Message is addressed to an object. Inheritance Definition of a class in terms of another class. OOP allows easy re-use and extension Software reused when used as a part of software other than that for which it was initially designed.