Define:
The mediator pattern defines an object that encapsulates how a set of objects interact. This pattern is considered to be a behavioural pattern due to the way it can alter the program's running behaviour.

Usage:
Components need to communicate with each other 
They communicate via the mediator 
The components all have knowledge of the mediator

Advantages:
Hides coordination between components 
Component no longer need to know about each other
Mediator turns many to many relationships into one to many/many to one.

Disadvantages:
The mediator can become large and very complicated