Computer network switches (the older, simpler, versions were called 'hubs') connect your networked computers together. You need a switch if you are going to have more than two pieces of equipment on your network. We have switches with from four to sixteen ports, in Gigabit and 100/Base-T speeds, and designed to sit on a shelf, mount in a structured wiring cabinet, or bolt into a computer rack.
Note that routers often have a switch built-in so that a separate switch is not necessary.
