A router is a computer networking device that forwards data packets between computer networks. It is connected to two or more data lines from different networks and decides which way to send each data packet based on its current understanding of the state of the networks it is connected to. Routers are used to connect networks of different sizes, such as connecting a home network to the internet. They can also be used to connect two parts of the same network, such as two buildings in the same city. Routers are essential for the functioning of modern networks.