A router is special device which is connected to more than one computer networks. Router forwards data packets between computer networks, creating an overlay internetwork.
A router is connected to two or more data lines from different networks.
When a data packet comes in on one of the lines, the router reads the address information in the packet to determine its ultimate destination.

Routers used in networks perform the following functions:
Restrict network broadcasts to the LAN.
Act as the default gateway.
Move data between networks.
Learn and advertise loop free paths.