What is the difference between 'in network' and 'out of network'?

What is the difference between 'in network' and 'out of network'?

In network is a managed care system where they list five doctors for you to go see. Often, those five doctors are either unavailable or they might not have the expertise that you need to fight your disease. The person you would then be seeking would be an out of network doctor, someone who is not in their network who has agreed to a reduced fee. Therefore when you go to an out of network, that doctor usually can request and receive a full fee for his services. The insurance company wants to limit those fees as much as possible, and doesn't like you to go out of network.