It frequently arises that the values of a type are partitioned into a variety of classes, each classifying data with a distinct internal structure. A good example is provided by the type of points in the plane, which may be classified according to whether they are represented in Cartesian or polar form. Both are represented by a pair of real numbers, but in the Cartesian case these are the x and y coordinates of the point, whereas in the polar case these are its distance r from the origin and its angle θ with the polar axis. A classified value is said to be an object, or instance, of its class. The class determines the type of the classified data, which are called the instance type of the class. The classified data itself is called the instance data of the object.

Functions that act on classified values are sometimes called methods. The behavior of a method is determined by the class of its argument. The method is said to dispatch on the class of the argument. Because it happens at run time, this is called dynamic dispatch. For example, the squared distance of a point from the origin is calculated differently according to whether the point is represented in Cartesian or polar form. In the former case the required distance is x2 + y2, whereas in the latter it is simply r itself.