A basic understanding of the difference between hubs, switches, and routers is important, given how they are pieces of equipment that frequently get added into a network in a piecemeal fashion. What follows might help you as you start down the aisles of Best Buy.
hub - used to connect 4 or 8 ethernet items together into one box so that you can plug them all into one ethernet port. Cheap and simple ($30?) It has no smarts - the signal from one device goes into the hub along its cable and is sent out on all 7 other cables. It gets processed by the one correct one and ignored by the others, although all of this adds to the total traffic. Hence, it is best to stay away from hubs.
switch - looks like a hub, but has some smarts, limiting the number of total signals bouncing around. Our IT department's recommended buy list tells us to buy $60 switches instead of the hubs to connect our 8 items to one port - presumably because of the limit of extraneous traffic. Switches can also be much bigger - our telecommunication closets have switches with 64 ports in them that cost more like $1000 each. But because the switches use hardware rather than software to control all this, they are quite fast.
routers - a lot of the same functions as switches, but with more of the functions done by software. Consequently, they are very powerful and also very complicated to set up. At a school network level, they need to be dealt with by people who know way more about this than me. But many of us deal with routers when we put two or three home computers on one dsl or cable modem box at home. The signal comes into the house through the modem, then goes by ethernet wire to the router (which also might have wireless built in, too). That router makes all sorts of decisions about what signal can go where - you can have your home computers talk to each other and share pictures, printers, music, etc. while computers outside the router cannot. When you first set up the router with the CD and the cheat sheet, you are programming a router.
A better summary of this than mine is found on Webopedia : http://www.webopedia.com/DidYouKnow/Hardware_Software/2006/router_switch_hub.asp