An image map is an image on a web page that is associated with a list of coordinates indicating specific points within the image. Each of these points is associated with a link to another web page, or to somewhere else on the same web page.
It's good to create a small piece of text to associate with each mapped point, so that when someone rolls his or her cursor over that point, they will see the text telling them what to expect when they click on that point. Visually impaired people will also benefit from this text, as their screen readers will be able to tell them something about what you've pinpointed in the image map.
There's a great example of an image map in the article about image maps.