Most computers have a single connection to some network and use that to communicate with "the rest of the world". The network normally has connections to a huge number of other computers so that, with proper authorization, each can exchange info with others on request. Many networks also includes a few other devices such as printers, scanners, cameras, etc. that can also be accessed by computers. Every device (computer, scanner, camera, or whatever), must have a network interface system that does all the work of sending and receiving information in data packets though the networks from one device to another. For this purpose, EVERY data packet sent out contains addresses of the source and destination devices, and every device "listens" to the data packets flowing past and actually captures only the ones with the right address. To help manage all this and NOT send ALL data packets to EVERY device in the network, the network has several data SWITCHES. Each of these has several ports to connections to devices and one or two ports for connection to this (or other) network(s). The switch constantly maintains and updates a list of what device addresses it has connections to, and of routes through network connection lines to other switches with their own connected devices. Each switch then also examines the address or each packet it receives and decides which output port to send it to so it can reach its intended destination.If it does not know which direction to send a new packet, it has a default location to send it that CAN figure that out and report back so the switch can update its info. In this way a Switch can ensure that every packet it receives is sent only to a network branch that will get it to the correct destination by the shortest route, leaving other branches uncluttered with unnecessary data packets.
An individual computer will have several peripheral devices attached directly to it for certain tasks such as keyboard, mouse, scanner, printer, camera, external hard drive storage unit, display, etc. Each of these is intended for use only by that computer and normally cannot be accessed by any other. In some cases, however, the computer MAY be set up to allow access to a few such "locally-owned" devices by selected other computers nearby and connected to a local small network. In these cases those devices still are "owned" and controlled by the computer they are attached to, and access is possible only by going though the computer that "owns" each such device.
Many times one person may have a need to use several computers at one location for different tasks at slightly different times. Rather than have each such system set up adjacent to each other and have the user shuffle back and forth between desks, it can be more convenient to arrange that the single user can access all of the computers from one location and use only ONE set of t he most important devices - keyboard, mouse and display - to control all these computers. BUT those devices can be used only with ONE computer system at a time. So we need a system to connect all three key devices as a complete set to each of those computers. This is the KVM SWITCH. KVM stands for Keyboard, Video and Mouse. For each computer, when it has NO connection to all of these devices, it simply does not attempt to use them. At any time the ONE computer connected to this set of three devices responds to them and the user has control of that one system. So a KVM Switch can make connections of one set of direct user interface devices to any of several computers in a manner that appears (to the computers) as perfectly normal, but is has nothing to do with devices far away from each computer on a network.
There is a third form of sharing access to peripheral devices common on local networks. Some types of devices are equipped with their own network access ports and can respond to requests for certain services received on from the network from other devices with proper authorization. Typically those might be printers, scanners, cameras, and maybe a data storage system containing multiple disk drives for general use. I'll use our home network as a simple example. Our house has a single fibre optic cable connection to the outside "world" which feeds into a Router (a more complex device than a Switch with more "smarts" to manage data traffic) that distributes data packets to several switches in the house. The "backbone" of the network is a set of cables between the switches plus four point-of-access devices that provide WiFi access for computers not connected by cables. Three computers in different locations all are connected by cables to switches - mine, my son's, and one on our TV that serves as its Digital Video Recorder device. My grandson's desktop system, my son's portable computer from work and my daughter's tablet unit use WiFi, as do all our cell phones and some smart switches for lights and a door opener in the garage. Thus all of these units have access to each other and to the outside world via the optical feed cable. My son's computer has an app running that constantly records images fed from some cameras around the house and yard, and many of us have apps on our phones to access those videos at any time. There is one colour printer/scanner connected to the network that can be accessed by any computer in our system. I also have a couple of other printers connected to my computer directly that are only available from this machine. We have permissions set up so that some hard drives on some computers can be accessed from other in-house computers. In the past (not now) I have had two computers set up at my desk and using a KVM switch to access both from one keyboard / video screen / mouse set. I'm pretty sure my son is dong this regularly with several systems in his room.
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