Networking is the practice of connecting multiple computing devices (such as computers {desktops, laptops and severs}, printers, data devices {NFS Servers}, sensors {remote sensing devices, GPS} etc) together in order to share resources.
Two endpoints are minimum, to create a network, as shown in the diagram below.
In the above diagram D1 & D2 are networked devices and red arrow is the networking medium. Multiple methods are used to create network such as cables, satellites, radio waves, infrared beams etc.
There are many types of networks available, most commonly employed networks are LAN, MAN & WAN.
Local Area Network (LAN): These networks are used to connect devices over relatively short distances, such as within a building, school, or home. LAN's generally employ Ethernet cables or WIFI as a means of connecting the various devices within the network.
Metropolitan Area Networks (MANs): These networks cover larger areas than LANs but smaller areas than WANs. These MANs generally span a city and are owned and operated by a government or corporations.
Wide Area Network (WAN): These networks are used to connect devices over much larger distances than LANs. A WAN is established by using routers to connect various LANs and are generally not owned by a single person or organization. Internet is one such massive WAN which covers the entire planet & satellites.