A Router is a networking device that directs the flow of data along networks. Routers are connected to at least two separate networks, commonly two LANs (Local Area Network) or WANs (Wide Area Network) or, as in your home network, your LAN and the Internet. Routers are also known as gateways, located where two or more networks connect, and are the critical device that keeps data flowing between the networks.
The routers we use in our home networks are sometimes referred to as “residential gateways” and are frequently used to connect to a broadband service over cable or DSL. These routers may also include an internal cable or DSL modem. Residential gateways typically provide firewall functionality through network address translation (NAT) and port address translation. Instead of directly presenting the IP addresses of local computers to the remote network, such a residential gateway makes multiple local computers appear to be a single computer. In terms of home network security, your router is the mask you wear, effectively hiding your real identity from the internet.
This article provides a very simplistic definition of a router. The router performs an amazing amount of work behind the scenes, and is the device that is most responsible for allowing the Internet to work at all.