Virtual Server (From the D-Link help file)

The device can be configured as a virtual server so that remote users accessing services such as Web or FTP services via the public (WAN) IP address can be automatically redirected to local servers in the LAN network. Depending on the requested service (TCP/UDP port number), the device redirects the external service request to the appropriate server within the LAN network

At the bottom of the screen, there are already defined well-known virtual services. To use them, click on the edit icon. You will only need to input the LAN IP address of the computer running the service and enable it.
Name - The name referencing the virtual service.
Private IP - The server computer in the LAN network that will be providing the virtual services.
Private Port - The port number of the service used by the Private IP computer.
Protocol Type - The protocol used for the virtual service.
Public Port - The port number on the WAN side that will be used to access the virtual service.
Schedule - The schedule of time when the virtual service will be enabled.
Example:
If you have a Web server that you wanted Internet users to access at all times, you would need to enable it. Web (HTTP) server is on LAN computer 192.168.0.25. HTTP uses port 80, TCP.
Name: Web Server
Private IP: 192.168.0.25
Protocol Type: TCP
Private Port: 80
Public Port: 80
Schedule: always