VLAN

• A virtual LAN (VLAN) is any broadcast domain that is partitioned and isolated in a computer network at the data link layer (OSI layer 2)

To subdivide a network into virtual LANs, one configures a network switch or router
VLANs allow network administrators to group hosts together even if the hosts are not on the same network switch.
Without VLANs, grouping hosts according to their resource needs necessitates the labour of relocating nodes and/or rewiring data links.
https://en.wikipedia.org/wiki/Virtual_LAN

• A VLAN is a group of end stations with a common set of requirements, independent of physical location. VLANs have the same attributes as a physical LAN but allow you to group end stations even if they are not located physically on the same LAN segment

VLANs are usually associated with IP subnetworks. For example, all the end stations in a particular IP subnet belong to the same VLAN. Traffic between VLANs must be routed
http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/vlans.html

VLAN Explained

isolating departments via LAN, finance dept uses seperate LAN

hosts must be on different switches in order to seperate LANs

virtual LAN, VLAN is  a logical network, logical segmentation

multiple networks/LANs on the same physical network switch

traffic from VLAN1 is logically seperated from VLAN2 inside the same switch, isolated traffic

VLAN logical segmentation, not physical segmentation

deploying 2 different LANs is the same as deploying 2 different VLAN on the same hardware

VLAN reduce HW: router-on-a-stick

interVLAN traffic requires a router or Layer3 switch

VLAN1 as default VLAN

Layer2 frame header contains VLANID(tag)

VLAN ports

port VLAN ID (PVID)

access ports (hosts, single PVID)

trunk ports, connect to other trunk ports on VLAN ready devices like switches,access points

trunk ports can carry untagged traffics if desired, one VLANID should be assigned to untagged traffic 

At Layer2 each VLAN is seperate broadcast domain(VLANID in frame header)

At Layer3 each VLAN is assigned IP range.