Computer networks allow computers to connect and share resources like files, printers, and communication tools. There are different types of networks including local area networks (LANs) within an office or building, metropolitan area networks (MANs) across a city, and wide area networks (WANs) spanning multiple cities or countries. Data transmission in a network can occur via simplex, half-duplex, or full duplex modes - with simplex allowing only one-way transmission, half-duplex allowing bidirectional but not simultaneous transmission, and full duplex supporting simultaneous transmission in both directions.