The RADIUS protocol abbreviates the name Remote Authentication Dial In User Service. The name suggests one typical use of RADIUS - for authentication, authorization, and accounting (AAA) of dial in modem pool connections. However, RADIUS is broader in application, also providing a AAA solution for wireless and other forms of network connectivity.
RADUIS is an IETF standard, available both in open source and proprietary systems. RADIUS removes AAA responsibility from edge devices such as routers or firewalls and instead requires the use of an authentication server located deeper in the network. The edge device then passes authentication request-response traffic between the client and the authentication server. (See also 802.1X.) This arrangement allows for authentication to be integrated on the back end with more robust directory management systems like LDAP, Active Directory, or SQL database applications.
RADIUS should use the assigned UDP ports 1812 and 1813. However, earlier versions ran on UDP 1645 and 1646, so port number settings should be verified, not assumed.
See: RFC 2865.
References:
http://www.tutorialspoint.com/radius/what_is_radius.htm
http://www.untruth.org/~josh/security/radius/radius-auth.html