Remote Access Service
Remote Access Services (RAS) refers to any combination of hardware and software to enable the remote access tools or information that typically reside on a network of IT devices. It refers to the authentication for remotes services access. A RAS server is a specialized computer which aggregates multiple communication channels together. Because these channels are bidirectional, two models emerge: Multiple entities connecting to a single resource, and a single entity connecting to multiple resources. Both of these models are widely used. Both physical and virtual resources can be provided through a RAS server: centralized computing can provide multiple users access to a remote virtual operating system. Access Providers often use RAS servers to terminate physical connections to their customers, for example customers who get Internet through some form of modem.
Originally coined by Microsoft when referring to their built-in NT remote access tools, RAS was a service provided by Windows NT which allows most of the services which would be available on a network to be accessed over a modem link. The service includes support for dialup and logon, presents the same network interface as the normal network drivers (albeit slightly slower). It is not necessary to run Windows NT on the client - there are client versions for other Windows operating systems.
To use RAS from a remote node, you need a RAS client program, which is built into most versions of Windows, or any PPP client software. For example, most remote control programs work with RAS.
Starting in the mid-1990s, several manufacturers such as U.S. Robotics produced "modem terminal servers". Instead of having RS-232 ports, these would directly incorporate an analog modem. These devices were commonly used by Internet service providers to allow consumer dial-up. Modern versions interface to an ISDN PRI instead of having analog modem ports.
See also 
Dial-up connectivity, not based on centralized control and least preferred from a security and control standpoint, is an organization's server whose operating system is set up to accept remote access, which is referred to as a remote access server (RAS). This is a very difficult method to control from an auditing perspective.
Routing and Remote Access Service
This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later. This article has some additional material from Webopedia