ok.....
Router are techinaly two devices in one. they are switches (see below) and dhcp servers. Dhcp gives a ip address to every client.
switches are a device that does not give out a ip but can obtane many ip addresses and forward it to the devices on them. they thus extend the lan letwork.
hubs are considerd dumb switches. they have no brain behind them and just pass traffic to every device and the one with the right ip will pick it up.
bridges are two devices that can link two networks together.
there is a lot more on this stuff but this is the bacics