Ok you can create internetwork by connecting your WANs and LANs to a router. It can be you have some hosts in your lan interface can be sayed as fastethernet port or interface.
Fastethernet interface ip address is called the default gateway for the hosts that are connected to this LAN i mean fastethernet interface.
Routing can defined as follows : taking a packet ( data ) from a device like host and sending it through the network to the other host or device that is located in different network.
Now router don't really care about hosts actually they care about the networks that are in their routing tables and selecting the best path for each network.
What things a router must know to route packets
1. router needs to know the destination ip address
2. router should know about its neighbor routers which router can learn about the remote networks and best path to go that networks.
3. it also need to know the all possible routes to all remote networks.
4. also need to know what is the best route or path to each remote network and how to reach that .
5. it also need to know how it can maintain and verify the routing information.