What is the diference between a switch and a router ?
A router essentially determines which way is the shortest or fastest in a network, and routes packets accordingly. It works at layer 3 of the OSI model, moving packets from one port to another based on L3 addresses - ie. IP addresses, IPX addresses, etc. A router (which often includes a built-in switch) allows you to connect multiple computers to each other (the switch part) and also allows them to share a single Internet connection (via the router part).
A switch connects one point to another in a network temporarily by turning it on and off as necessary. It works at layer 2 of the OSI model, with some intelligence (there are also some layer 3 switches, that essentially have routing capabilities). A switch only allows you to connect multiple computers into a local network.
Note: you can expand the number of LAN ports on a router/switch by connecting another switch to it with a crossover cable or via an uplink port.