Your internal infrastructure is analogous to the heart and veins of your network. Your physical devices such as switches routers firewalls and servers, supplying the structure to support the blood flow of the system, your data.
This system is made up of various networks described as either a LAN, a local area network, or a WAN, a wide area network. Local area networks (LAN) generally span a relatively small area. Maybe a building or a room in a building, allowing servers, workstations, printers, and people to share resources.
Wide area networks (WAN) generally span geographical areas such as towns, cities, or even countries in some instances. Connecting various LAN networks to one another. In between these various networks you could have switches, routers, and firewalls.
Switches being the aggregator of the systems on the network (servers, printers, workstations), connecting them to one another.
Routers are tasked with directing connections over the WANs or LANs, dictating the most efficient route to take to any given resource.
The firewall is a sort of gatekeeper if you will, sitting between the networks, allowing or denying access to resources across these networks.