What is meant by RAS server & IAS server? what is the use of this server?

RAS (Remote Access Service) is a Windows Service that allows users to access the Organizations Resources Remotely.

IAS (Internet Authentication Service) is a Microsoft Implementation of Remote Access Dial-In User Service. It allow to a mechanism for Authentication, Authorization, Accounting and Auditing.

As a Radius Server,  IAS can be used to authenticate, authorize, Account and audit clients for many connection types like Wireless Service, VPN, Remote Access Dial Up, authentication switch, etc

As a Radius proxy, IAS works as central switching and routing point for Radius access and accounting messages to flow between Radius Clients, Radius Servers and Radius Proxies.

You use these servers specially when you are trying to connect from a remote place, I think RAS is a advance release of Remote Authentication Dial In User Service (RADIUS). 

Remote Access Server (RAS) - As the name says "Remote Access), this will allows users to gain access to files and print services on the LAN from a remote location.

Internet Authentication Service (IAS) -  IAS performs centralized connection to4As (Authentication, Authorization, Accounting, and Auditing (AAAA). IAS performs4As for different types of network access which includes VPN and wireless etc connections.