Virtual LAN?

Hi Guys
 
Can anyine explain to me how to use VLAN in monowall. Do I need a 3rd LAN
CARD? What is it actually for? I have read few description over the internet
and it seems that VLAN is for creating a network or a group of computers
from different Switches, IP pool etc and make them think they are in one
same IP Pool so they can communicate by using TAG.
 
So, what is m0n0wall's position in this network. Do I attach 3rd LAN Card
into the m0n0wall PC and then connect to a new Switch, so it becomes like
the second LAN??
 
This question came up to me because I was asked to create 2 zones of
internet users where 1 zone has full priviledge and the other is limited to
just intranet. No chat, no browsing, etc. Then my boss told me someone told
him about setting up a VLAN.
 
I am confused. Please enlighten me :)
 
Thanks
Rendra