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A switch and a hub are almost the same thing.  The difference for a business is pretty important, but for a home is usually irrelevant.  Hubs are rare these days anyway, so we will just discuss switches, the better of the two.  The information is the same for both.

A switch is essentially a "network splitter".  The only way to split a network cable into multiple network cables is with a switch (or a hub).  A network using a switch MUST also have a router (there are exceptions, but if you knew that you likely aren't reading this, so it's unimportant).  All a switch does is turn one port of a router into multiple ports.  The way it does this is you connect one of the output ports from a router to one of the ports on a switch, then the rest of the ports on the switch essentially become output ports for the connected router.  It is rare these days, but sometimes the router must be connected to a specific "uplink" port on the switch.  Newer equipment generally detects the uplink automatically, regardless what port you use.

I said that a switch is essentially a network splitter, but that isn't entirely accurate.  It's actually a repeater.  A signal comes into the switch and then the switch reproduces that signal on another port.  It sort of "passes along" your message, but it's not the original communication which is passed, it's a reproduction of the communication.  Because of this a switch will "boost" a signal.  As long as it gets an accurate signal in the first place that signal will be faithfully reproduced and sent out again at full power, as if it were the first time it was being sent, without any degredation of the digital signal.

A switch does not create a network, nor does it assign addresses.  These are both functions of a router.  If you connect a switch directly to your modem your network won't work.  A modem has a single address, remember.  Multiple devices on your network cannot have the same address.  That is why a router creates a whole new "internal network" and links it to the single Internet address assigned.

There is one thing you can do to a switch which will immediately take down your network.  If you have a switch which doesn't appear to be working, trace all the lines coming out of the switch.  If you connect an Ethernet cable from one port in a switch to another port in that same switch, or if you connect multiple lines from one switch to a router or to another switch it will take down the network.  A switch doesn't know how to handle communications with itself or multiple lines of communications with the same router or switch.