What is the difference between an internal and an external method call? In what situation would only internal calls be needed?

The difference between internal method call and external method call are as follows:-

Internal Method calls means when the function is called using the object of the class in the same class which is defined but where as in case of external method, it calls the function are called from other class by creating the object the respected class in other class and the function is called.

Suppose there is only one class and that class is only having all the methods and function declaration and there is no other class described. So in this case we will use the internal Method calls only.