The polygon shapes for the hot spots are described as a string of x y coordinates. The text of this list of coordinates may be located within the body of an HTML document , this is called a client side image map , or stored on the server as a separate text file, this is called a server side image map . This overhead can make this method slower than client side maps. Client side image maps only require adding a map tag to the image source tag and the coordinates for the hotspot shapes. This methods is faster since all the information to activate a link has already been loaded into the browser and is just as fast as a textual Hypertext link.
Image maps work only when the image is downloaded and viewed.
So if a user has images turned off, is using a text only browser, or is visually impaired, the image map links will not work. It is a good idea be sure of your audience needs or provide an alternate navigation method that will still work if read aloud by a speech synthesizer. Below is an example of a client side image mapped image.