What is the right/specific terminology for this whole topic/subject, so that I can google it.

I think I'm getting it already....So its like everything has to go through the firewall pc. Its like a filter bag. Correct me if I'm wrong. Thanks!

Case Scenario:

Assuming the ff will be utilized:

Red (external Internet)
Orange (DMZ or DeMilitarized Zone)
Green (internal network)
Blue (wireless network)

Example:
For internet access (from ISP), the connection is:
ISP (modem)--->WAN port (Router #1)--->firewall pc (ethernet#1)
and if DMZ is utilized
firewall pc (ethernet#2)--->(Router #2)--->web servers, VOIP, etc.
and for the internal network
firewall pc (ethernet#3)--->(Router #3)--->workstation(s) #1, #2, #3, etc
and finally the wireless network
firewall pc (ethernet#4)--->(Wireless Router)--->workstation(s), laptop, portable devices, etc..