When and how do you use DIV tags?

I use DIV's to divide content so that, for example, you could have a hidden div that is shown at some point (such as an onclick etc.).:
http://www.netlobo.com/div_hiding.html

<div> tags can be used whenever you need a special block of tags to be considered as a unit. You did not use any in your site, because all you needed were tables and an image map. Nowadays, website designers are striving to use CSS and <div> tags to layout their sites, but tables are still around and generally work.

<div> really just stands for division, so it is up to you when you want to use one. You may want to enclose your form in a <div> block. This gives you the liberty to style the <div> with CSS. Maybe you can absolutely position it over your image. Of course, you could just put a style on the <form> tag itself. <div>s are most useful when you have several tags to which you want to apply the same style.

Ask your backend guy what he needs. He should be able to tell you if he specifically needs <div>s for any reason.

"where can I put div tags so that my backend guy's job is easier when implementing forms in the asp pages. "

Makes no difference for backend guy.  DIV are primarily used for style and design, with CSS.    You SHOULD tell your desiger to produce it with CSS rather then tables.