what is Virtual Lan (VLAN)

Virtual Local Area Network, Virtual LAN or VLAN are network of computers that behave as if they are connected to the same wire even though they may actually be physically located on different segments of a LAN. VLANs are configured through software rather than hardware, which makes them extremely flexible. One of the biggest advantages of VLANs is that when a computer is physically moved to another location, it can stay on the same VLAN without any hardware reconfiguration. In the following picture :

A) 1,2,3 configured for a single physical network (in a area or building), but it’s a combination of VLAN 1 & VLAN 2
B) 4,5,6 configured for a single physical network (in a area or building), but it’s a combination of VLAN 1 & VLAN 3
C) 7,8,9 configured for a single physical network (in a area or building), but it’s a combination of VLAN 1, VLAN 2 & VLAN 3.
 
NOW
 
If someone, hopefully administrator deletes VlAN 1 then connectivity will be lost in following terminals (shown in black):