This tutorial assumes a typical home broadband network, not the more complex corporate one.
The purpose of a router is to join a WAN (internet) to a LAN (home network).
But routers targeted for SOHO (small office/home) network already include a built-in switch (see the difference between router and hub/switch) so non-technical users can simply plug their computers to the router without worrying about the technical stuff.
From a wired hub/switch, you can attach a wireless access point so you can have WiFi. But then, there are also wireless routers so you can go up and running in no time.
If you get deeper than the above diagram, here’s what you get:
There are many possible combinations but the pattern remains for typical home networking:
MODEM (WAN) -> ROUTER (LAN) -> HUB/SWITCH -> COMPUTER
Note: Switch is to wired networking what wireless access point (WAP) is to WiFi.