A router is a networking device that connects different networks together and allows communication between them. It uses logical addressing like IP addresses to direct traffic between the networks. The document discusses different types of routers from Cisco including access layer routers for small networks, distribution layer routers for ISPs, and core layer routers for large backbones. It describes the various ports on a router like Ethernet, serial, console, and auxiliary ports and their purposes. The boot process of a router is also summarized where the ROM loads a bootstrap program from flash memory which then loads the IOS software and configuration from NVRAM into RAM.