RADIUS Stands for (Remote Authentication Dial In User Service)
RADIUS enables centralized management of authentication data, such as usernames and passwords.
When a user attempts to login to a radius client, such as a router, the router send the authentication request to the radius server. The communication between the radius client and the radius server are authenticated and encrypted through the use of a shared secret, which is not transmitted over the network.
RADIUS can significantly increase security by enabling the centralization of password management. Of course, the other side of that argument is that once you take over the RADIUS server, you have everything.
RADIUS is the de facto authentication provider in 802.11i wireless networks.