wan vs lan

What are the differences of having ethernet cable from a router plugged into one of the TC LAN ports instead of into the WAN port.

Nothing, if your TC has been configured to operate in "Off (Bridge Mode)", also known as Bridge Mode, which is the correct setting for it to work with an "upstream" router.

Is the TC in "Off (Bridge Mode)"? It should be if it is connecting to another router.

In Bridge Mode, all ports are the same. They all function as LAN <--> ports. In other words, there is no WAN "O" port when the TC.... or another Apple router.....is set up in Bridge Mode.

IF.......your TC was connected to a simple modem.....(the kind with only one Ethernet port).....then the TC would be configured to operate as a router and not a bridge. The modem must connect to the WAN "O" port in this case, since the WAN (Internet) port will handle an Internet connection from a simple modem.

WAN is Wide Area Network. LAN is Local Area Network. That gives users a pretty good idea why they're separated.

If you have a cable or DSL modem that is just a modem, and need to plug it into your router, you use the WAN port to connect the two as you are communicating with the wide network of your service provider, and the Internet in general.

You use the LAN ports for anything using a wired Ethernet connection to the router in your local space. Meaning, inside your home or business. Which can be any computer, an Ethernet capable printer, a small NAS (Network Area Server), etc.