Okay, I was going between "consumer" and "newb who got thrown in the deep end" ...
PVID = Port Virtual ID.
This helped me wrap my head around VLANs
Scenario: You have three physical networks, red, blue, and green (each cabled with relevant colors, and switches marked with r/b/g stickers, etc.)
You want to simplify things, so you buy a router and switch(es) that can do VLANs ("Virtual" LANs).
You set things up with three VLANs - 10, 20, 30 - which correspond to Red / Blue / Green.
Now, the port between the router and the switch is set up to carry all three networks (i.e. tagging vlans 10/20/30). So you get a WHITE LAN cable, and some R/B/G sharpies, and color r/b/g stripes on it.
For the rest of the hosts, in order to keep them on the proper network (red, blue, or green), you set the port PVIDs as necessary.
For example:
"Red" Network - 12 hosts / set ports 2-13 as PVID10 (use red cables again, etc.)
"Blue" Network - 6 hosts / set ports 14-19 as PVID20 (use blue cables again, etc.)
"Green" Network - 4 hosts / set ports 20-23 as PVID30 (use green cables again, etc.)
Edit -- oops this bit got cut off...
Without talking to the router, all the hosts on each colored network think that they're the only hosts in the building. The VLAN tagging, enforces the separation between each network and prevents anything from communicating in ways that aren't desired.