can someone please tell me what are the differences between Wireleless Distribution System and Virtual Access Point???
Roughly the same difference as between an Orange and a Baseball Bat.
WDS is a system whereby two access points can be set up to allow them to pass traffic between them as if one was an AP and one was a client. This then extends the traffic from one AP to another, allowing links of WDS enabled APs to pass traffic amongst them. Often used for extending the range of an AP so a client can communicate to another client via multiple APs.
Virtual Access Point is the ability of a single AP to operate with a whole new SSID, and allow a completely new service to be operated over it. In this way one may operate multiple SSID's with perhaps one operating as a Wireless Hotspot with no encryption and another Virtual AP with another SSID operating as an WPA encrypted service with no Hotspot. Becuase they share the same real hardware wireless interface, although the AP will broadcast different SSIDs they will always all be on the same frequency.
I could go into more detail, but that's very roughly what the difference is!