A Router is a device that connects two networks - frequently over large distances. It understands one or more network protocols, such as IP or IPX. A Router accepts packets on at least two network interfaces, and forwards packets from one interface to another.
Router's may be programmed to filter out some packets, and to dynamically change the route by which packets are routed.
Router's often use different media on each interface. For instance, a router might have one Ethernet port and one ISDN port.