An image map allows a person to click on different parts of an image and link to different data. This is useful if you want to diagram an object and allow the user to click on each part to get more information. Here is an excellent example of an image map of the human skull . Various applications will create these image maps for you by plotting the the numerical x,y coordinates of the point. The server computer processes the image map and links to the data.
Most browsers support server side image maps.
A client-side image map includes the numerical x,y coordinates right in the HTML code of the document. The browser does not have to connect with the server computer to read the image map. It is much faster than a server-side image map but not all browsers can read them. So, it might be a good idea to include both kinds of image maps. You can make any image into an image map using Claris HomePage or Adobe Pagemill.