A LAN (Local Area Network) is a group of devices that are linked together in the same physical location (i.e, your home, an office building).
Network switches connect devices within the same network. Large businesses often have a separate switch, but most household internet setups use a router with a built-in Ethernet switch. Ethernet is the most common way devices are connected together on a LAN.
A WAN (Wide Area Network) often connects many LANs together.