The first Web authoring program that supports dHTML comes from Macromedia and is called Dreamweaver (currently available from www.macromedia.com in beta). It comes with wonderful tutorials on dHTML, what it is, and how to use it. The following is the overview of dHTML that comes with DreamWeaver ....
The ability to change style or positioning properties with a scripting language is what is commonly called dynamic HTML, or DHTML. Dreamweaver has four DHTML-related features: Layers, Styles, Timelines, and Behaviors.
The term "layer" was introduced by Netscape to refer to its LAYER and ILAYER tags. In Dreamweaver, "layer" means any element that can be positioned at exact coordinates on the page. Positioning properties include left and top (x and y coordinates, respectively), z-index (also called the stacking order), and visibility. Positioned elements can be defined with the DIV, SPAN, LAYER, or ILAYER tags. Positioned elements created with DIV and SPAN are commonly referred to as "CSS layers" or "CSS-P elements" because their positioning properties are defined by the World Wide Web Consortium's Cascading Style Sheets Positioning specification (CSS-P). Both Microsoft and Netscape support CSS layers in their 4.0 browsers. Positioned elements created with the LAYER and ILAYER tags are supported only by Netscape Navigator 4.0 or later.
Here are some places to go to learn more
All you need to know about dHTML for Netscape4: http://developer.netscape.com/library/documentation/communicator/dynhtml/contents.htm
Alphabetic Listing of HTML tags from Netscape (includes div, layer, and other dHTML tags): http://developer.netscape.com/library/documentation/htmlguid/alphlist.htm
The W3C (World Wide Web Consortium) Page (the folks who write the protocol): http://www.w3c.org
Microsoft's take on dHTML: http://www.microsoft.com/msdn/news/feature/110397/dhtml/default.htm