@Derelict:
Inherited a network and you're not a network guy, huh. Must not be very important to TPTB that their network actually work.
Artecs, don't worry. You don't need to be a network guy to setup pfSense but it helps if you are willing to learn what you do need to know.
VLANs are far easier to conceptualize if you understand why they exist.
To expand on robi's comments…
You have 5 LANs. In the old days, you would need 5 physical ethernet interfaces in your firewall to service them.
VLANs enable you to collapse 5 physical networks into just 1 physical network so that only 1 physical interface is required to service them all. This cuts down on cabling and hardware and can make remote moves and changes much easier.
VLANs do this by tagging packets with the label that you assigned to them so that they can be identified and separated later.
If you have a physical cable plugged into a pfSense ethernet interface that is running one or more tagged VLANs, the other end of the cable should be plugged into a tagged port on a VLAN switch. It is usual to make this port a member of each VLAN that it is servicing.
If you have a physical cable plugged into a pfSense ethernet interface that is not declared as a VLAN, the other end of the cable should be plugged into an UNtagged port on a VLAN switch. This port only needs to be a member of the one LAN that it services. Alternatively you could just use a regular Non-VLAN capable switch or even a hub!
I hope this helps.