networking - Expanding wireless coverage: What are the differences between LAN to LAN and LAN to WAN when it comes to connecting two wireless routers?

I'm trying to link two wireless routers with an Ethernet cable in order to expand my wireless network. Both routers need to provide wired and wireless Internet access.

I'd like to know the differences between the "LAN to LAN" and "LAN to WAN" connection methods, their advantages and disadvantages.