Radius is a method of authenticating users and computers that are connected to a host system or network remotely or using wireless networking.  It can be used to control authentication and data encryption protocols on a wireless network system. 

RADIUS (Remote authentication dial-in user service) is a service that can be run on a Windows Server or Linux based machine.  It's primary role is to authenticate user credentials and to provide an access token that allows a user to gain access to networked systems.

Schemes like EAP, PEAP, CHAP or MS-CHAP are used to verify usernames and passwords against an Active Directory or other Network access control server.

When combined with a Windows based server, Radius can be used to provide a totally secure wireless system where access keys are controlled by the network.  Essentially this means that computers that are part of a Domain will be able to access WPSK enabled wireless network that is part of the same domain.  No security keys are needed, as the computer account has the ability to authenticate against the domain.  Once authenticated, users will be able to access networked resources as if they where connected to a hardwired cable.

There are many articles dealing with the setup of a Radius server for both Windows Server and Linux.  Microsoft calls its Radius server "Internet Access Service", this can be installed as a Windows Component from the Add/Remove Control Panel.