An image map is an image that acts as a link, but instead of simply acting as a link to a single page, the image acts as a link to multiple pages. For instance, say you have a map of the United States that you wish to act as an image map. Then you could specify different areas of the map to take you to different pages.
An image map consists of two parts. First, there is the image itself. This can be any image, so you don't need to alter the image in any way to make it act as an image map. The functionality to point to different links comes from within the HTML code itself.