Object-Oriented Programming is a programming languages different than old proceddural programming languages( C, pascal, etc.) In OOP, there have four main cpncepts: objects, class, inheritance, interface. First, "objects" is an  instance of a class, it can be considered a thing in an activites. Second, "class" is representation of a type of object. Third, "inheritance" is the ability of a new class to be created, from an existing class by extending it. Last, "interface" is a contract between a class and the outside world.  When we learning this, we had to understand the concept and purpose, it can let us easy to application.