From LQWiki

VLAN = Virtual Local Area Network You create a VLAN on a network switch. For example, if your network switch has 24 ports, you may want to allow the first 12 to see each other's incoming traffic, but to deny that to the other ports. That has the disadvantage that a computer connected to port 1 cannot communicate with one connected to port 13. But is has the advantage, that network sniffing is also impossible between those computers.