The essential device for connecting computer networks is the router. A router directs data packets between different networks, allowing multiple devices to communicate with each other efficiently. It connects local area networks (LANs) to the Internet and manages traffic by determining the best paths for data transmission. Routers can also have integrated firewall features for added security and may include additional functionalities like Wi-Fi access points. Overall, routers are vital for enabling network connectivity and facilitating seamless communication in both home and enterprise environments.