People connected to one hardware often want to communicate 
with people attested to a diffrent one . This desire 
requires connecting togather diffrent and frequently 
incompatible network. Sometime by using machines call  
gateways to make the connections and provide the neccessary 
translation both in terms of hardware and software.

A gateway is a network point that acts as an entrance to
another network. On the Internet, a node or stopping point
can be either a gateway node or a host (end-point) node.
Both the computers of Internet users and the computers that
serve pages to users are host nodes. The computers that
control traffic within your company's network or at your
local Internet service provider (ISP) are gateway nodes.

a computer or a network that allows or controls access to another computer or network