What is a Network, Really?

A network is just a set of devices that can send each other data. Every device on a network gets an IP address, a numeric label like 192.168.1.14 that works like a postal address, so traffic knows where to go. Data doesn't travel as one big chunk, it's broken into packets, small pieces that each carry a bit of the data plus addressing information, sent separately and reassembled at the other end.

A router is the device that actually moves packets between networks, deciding the next hop for each one based on its destination address. Your home router does this between your devices and your internet provider; the internet itself is really just an enormous chain of routers doing the same thing, hop by hop.