Since the 1980's a popular approach to programming has emerged known as Object Oriented Programming. This is a very powerful, yet simple, idea. In traditional approaches to programming there was a clear distinction made between data (that to be operated on) and program (the process that would operate on the data). The Object Oriented approach dispenses with this division.
An object is composed of both the data that describes it and the code that will operate upon it. As such, every object has within it everything it needs to go about its business. If an object is to be drawn it will draw itself. It will contain its own code for how to do that; it will not need to refer to or be acted upon by an external program.
That is the basic concept underlying Object Oriented thinking. However, there are a number of other very important concepts that accompany this approach.
Messages are the means by which objects communicate with one another. They have no other means of accessing one another internally. When designing an object your first thought is to what relation it will have with the world beyond itself. It will be in the nature of these relations that the object will come into being. The means by which the object is communicated with, and how it communicates with that beyond itself, is messaging. An object can only send or recieve a message if it has a bit of code (a message handler) incorporated into its design that is designed for a particular message. This concept of "privacy" is called "encapsulation".