Internet and its applications are an integral part of our daily life .These days they are widely used for various purposes such as communication, public services, entertainments, distant educations, etc., each possessing different quality of service (QoS) requirements. How to provide finer congestion control for network emerges as a major problem. To prevent the problem of congestion control and synchronization various active queue management (AQM) techniques are used. AQM algorithms execute on network routers and detect initial congestion by monitoring some functions. When congestion occurs on the link the AQM algorithms detects and provides signals to the end systems. Various algorithms have been proposed in recent years but RED is one of the most influential techniques among all the existing ones. This review paper provides the functioning mechanism of the RED technique with the help of its algorithm & its variants.