OSPF requires that each nonbackbone area be connected to Area 0. sometimes however this may prove difficult and this may not be achieved properly. OSPF virtual links can potentially be a workaround for this kind of problem, where an OSPF area, can only connect to the backbone, if it passed through another nonbackbone area.
a Virtual link allows a pair of possibly remote routers to create a targeted OSPF session across the IP network.
Virtual links are not tunnels but rather a targeted session that allows both neighbors to establish neighborship with each other and sync their LSDBs.’
Areas through which the session for the Virtual Link is created are referred to as transit areas…. and it must be a regular area, basically one that isn’t stubby in any way.