What are you going to do if your lovely router that you just bought and hooked up your whole network to ran out of ports?
Let's say I have many, many computers in my house, or in my office, and I connected all of the ports that I have here. Most routers will have four ports on them, so I have connected three computers and a printer. But suddenly I wanted to buy another computer. So, that's what the switch is for. A switch is basically kind of like a network surge protector. Just think about it this way. You plug in one output to your router, and the other ones will just split up that connection, to give you more hookups. This is called a four port switch, which will mean it will have actually five ports - one which will be uplink. It will send the signal back to the router - and the other four will split it up for four computers. Switches come in many sizes. They can come in four port size A ports, 16, 32, even 64. And of course, they have much larger more industrial switches for big industrial settings. And they also have different speeds to them. And when I say speed, I'm basically talking about the speed of the port. The basic speed is 10 megabit per second; then you have 100 megabit, which is pretty much the standard nowadays. And today, we also have faster switches which are gigabits. So you have 1,000 megabits per second. If you buy a gigabit switch, it's also wise to make sure your computer has a gigabit Ethernet port, and also that your router has a gigabit Ethernet port, otherwise you're not really thinking that that would do it.
So, if you have way to many computers for your router to handle, an Ethernet switch is definitely what you need.