Difference between an ECN and an exchange?

What is the difference between an ECN and an exchange?
ECN is an Electronic Communication Network ,An electronic system that attempts to eliminate the role of a third party in the execution of orders entered by an exchange market maker or an over the counter market maker, and permits such orders to be entirely or partly executed.
An ECN connects major brokerages and individual traders so that they can trade directly between themselves without having to go through a middleman. Sometimes ECNs are also referred as dark pool.