Adds dynamically new behaviors to an existing object without changing its inferface.
An alternative way to inheritance but more flexible because you can add new behaviors at runtime.
Example:
For instance we have computer to which we connect an USB device, computer is still a computer
(the same interface) but it works differently and allows us to perform other functions.
Once we remove a device form USB port the computer is no longer so cool.
A device might be connected dynamically, meaning while the computer is on - we don't have to turn
the computer off in order for a device to work. For instance a pen drive adds new disc and therefore
extends computer's memory.
