A router is a networking device that forwards data packets between computer networks. It operates at the network layer of the OSI model and is typically used to connect multiple devices within a local area network (LAN) or to connect a LAN to a wide area network (WAN), such as the internet. Routers use routing tables and protocols to determine the best path for forwarding data packets from the source to the destination. 

They also provide functionalities like network address translation (NAT), firewall security, and quality of service (QoS) management. Routers play a crucial role in enabling communication between different devices and networks in modern computer systems.