A gateway is a hardware device that acts as a "gate" between two networks. It may be a router, firewall, server, or another device that enables traffic to flow in and out of the network.
While a gateway protects the nodes within network, it is also a node itself. The gateway node is considered to be on the "edge" of the network as all data must flow through it before coming in or going out of the network. It may also translate data received from outside networks into a format or protocol recognized by devices within the internal network.
A router is a common type of gateway used in home networks. It allows computers within the local network to send and receive data over the Internet. A firewall is a more advanced type of gateway, which filters inbound and outbound traffic, disallowing incoming data from suspicious or unauthorized sources. A proxy server is another type of gateway that uses a combination of hardware and software to filter traffic between two networks. For example, a proxy server may only allow local computers to access a list of authorized websites.
NOTE: Gateway is also the name of a computer hardware company founded in the United States in 1985. The company was acquired by Acer in 2007 but still sells computers under the Gateway name.