When a user makes a selection within the image map, the browser will send the pixel coordinates of the user's selection to the provider's web server, which will run the image map program to look up those coordinates in the map file and return the appropriate URL you have bound to that selection. There are drawbacks and trade-offs to consider when using image maps as menus. First, remember that the Web is a platform-independent publishing medium. Some of the visitors to your page may not have graphical browsers at all, and some may have images turned off in their graphical browser to save time by avoiding image transfers over slow low-bandwidth connections. For those without image display capabilities, your image map is going to be completely useless because they won't have access to any of your links.
Text is still the fundamental communication medium on the Web.
If your site is intended for general access, even if you choose to use image maps, it is wise to also have a text-based method of navigating the image map links. One common technique is to include a redundant menu of hot-linked text above or below the image map menu. Another method is to include a link near the entry point of your web site that lets the user select a graphical or textual approach for subsequent pages. Or you can simply decide to ignore any text-based browsers. If you are sure of your audience, and you know they will have graphical browsers and reasonably high bandwidth, then you need not provide alternatives for your image map menus.