In OSPF all areas in the same AS must be physically connected to area 0 so a router (ABR - Area Border Router) that connects area 0 to area 10 for example would have at least one interface in area 0 and one interface in area 10.

Virtual links are used when there is no physical connection to area 0. See this attached doc for more details

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094aaa.shtml#virtuallinks

Note that virtual links are not good design. They are usually used as a temporary stop gap.

Jon

Your question is not totally clear. The basic OSPF rule is that all areas shall be connected to the backbone. If you have an area that does not have a direct connection to the backbone, to allow connectivity to other areas you would need to configure a virtual link.

The rule of thumb for virtual links? They are for emergencies, temporary use or for people that cannot design networks.