Simply put, a router connects two different networks
, with two different address spaces, and "routes" traffic between these networks.
A switch, on the other hand, connects devices to one network
You may have also heard of hubs. They are similar in function to switches with the exception that hubs essentially share one "lane" and switches have dedicated "lanes" for each port.
Hubs, as a result can have "collisions", whereas switches don't.
Nowadays, switches are so cheaps so there is no need to use hubs.
A good technical discussion on the difference between hubs and switches can be found at http://www.duxcw.com/faq/network/hubsw.htm
Wikipedia also has a good discussion about routers and switches at http://en.wikipedia.org/wiki/Router
Hope this helps!