A router is an important component in a computer network, because it facilitates the movement of data, or "packets," between devices and other networks. A router can also provide an additional level of security in a network.
What is a Network
When a router is connected to a modem, all of the computers that are connected to the router, and subsequently accessing the Internet through the modem, are part of the network. One router can connect nearly any number of computers, from a small home network of two or three machines to a large network of hundreds of servers.
A router essentially acts as a gateway between two networks. It has two main purposes. First, it makes sure data is directed to the correct destination, such as an email being sent to the correct Internet provider and recipient. Second, the router prevents data from going where it is not permitted, such as a large file being distributed to all machines on a network and crippling network performance.
The router acts as a buffer between the network and the modem. The router is not just a physical barrier for the Internet signal to pass through, but it also can have software security to diminish the risk of viruses or other malware.