Virtual Server, also referred to as Port forwarding or port mapping can be setup with the router.
The setup of the virtual server depends on what you want to setup.
Virtual server is used to allow computers not on your internal network or computers on the internet to access a resource or service on the internal network.
Say you have an FTP on your internal network that you want people on the internet to access. To set that up go to the virtual server screen on the router and specify the IP address of the computer on the internal network that has the FTP server on it. The private port that you want to specify would be 21 because port 21 is FTP. The public port could be any number you want it to be, for ease of explanation lets just say it is also 21, that is the port the public would use to access the machine. For a computer on the internet to access the FTP server on the internal machine they would open up a web browser and type FTP://(WAN IP ADDRESS, found on main status page of router screen):21.
A better example would be FTP://220.127.116.11:21
They could also use an FTP access program and use the WAN IP address then in the program would specify the public port.
You could have more than one FTP server at a time running through the router if you like. They would each of course have their own IP address that you would specify in the Virtual Server setting screen, the Private port would be the same, 21 for FTP but the public ports would have to be different so that the public, people on the internet, could distinguish between the two servers.