oInv.Display && Show the Invoice
Calling a method can be termed as sending a message to the oInv object to execute its method called Display.
If we understand the concept of a message, a great deal of the gobbledygook we read in OOP literature becomes understandable. For example, Polymorphism has been defined as "The ability to send the same message to different objects and have different actions take place." Translated into English, this means that I can have the same method name in multiple objects that do different things.
The moral of this story is: don't let the language throw you.