You use the Slice Tool. Simply slice up the image how you want it to be. Then adjust the properties of each slice and put in any needed html code. For example, if you sliced up an image into 3 pieces and you wanted each peice to be a link pointing to a different web page, then all you'd have to do is enter the target url in each slice's properties.
Then when it is all set, do SAVE FOR WEB from the FILE menu. Then you can just save all slices. Photoshop will make the HTML document for you (or it'll only make the HTML table if you so choose) and it'll save all the individual sliced up images in a sub-directory under wherever you saved the HTML document.
It's pretty slick. If you want to see an example of a multi-sliced up image with many various links then check out my fairly new (sorta)portfolio page here:
http://o.i.c.home.comcast.net/images/001.html
All I did was put all those images together as one image in PS...then sliced it up and saved it. PS did all the work of making the HTML table and getting everything to line up and look good.