That's a great question, one that I admit I had a lot of trouble finding
a straight answer to. I remember seeing a posting on a JavaScript newsgroup
that attempted to answer this question with the reply:
"DHTML is the combination of HTML and JavaScript"
I said to myself "Isn't that just a webpage with JavaScript on it then?"
I was confused, to say the least. After shuffling through hundreds of search
results on search engines for that elusive answer to my question, I still
could not find it. So I thought, what the heck, I'll learn DHTML first,
then figure out what it is! Now that I'm somewhat a DHTML programmer, I
think I'm ready to present you with a decent one-line definition to the
word DHTML:
"DHTML is the combination of several built-in browser features in
fourth generation browsers that enable a web page to be more dynamic"
You see, DHTML is NOT a scripting language (like JavaScript), but merely
a browser feature- or enhancement- that gives your browser the ability
to be dynamic. What you really want to learn is not DHTML itself, but rather,
the syntax needed to use DHTML. Before anything else, you may want to quickly
visit Dynamic Drive 
to see what this language is capable of.
Like I said, DHTML is a collection of features that together, enable
your web page to be dynamic. I think its important now to define just what
the creators of DHTML meant when they say "dynamic". "Dynamic" is defined
as the ability of the browser to alter a web page's look and style after
the document has loaded. I remember when I was learning JavaScript, I was
taught that you could use the document.write() method of JavaScript to
create webpages on the fly. For example:
<script>
document.write("This is text created on the fly!")
</script>
"Not bad", I remember saying to myself. But what if I wanted to create
content not only on the fly, but on demand? Naive I was then, I
tried doing just that, by nesting the above code in a function, and calling
it via a form button:
<input type="button" onClick="writeconent()"
value="text">
Pressing the button was nothing short of a big disappointment. My entire
web page was erased, and I was left with only the text the function produced.
That was back then. Now, with the introduction of DHTML, I can alter
content on a web page on demand, whenever I bloody feel like it, without
having the browser erase everything else. That's what DHTML is all about.
The ability of the browser to change look and style even after the document
has loaded.
Now that I've got you all excited, I think it's only fair that I put
a damper on it. The technology of DHTML is currently at its development
stage, with NS 4 and IE 4 differing quite greatly in their implementation
of this great technology. Its currently not possible to write one DHTML
code and expect it to function in both browsers properly. Furthermore,
the two browsers are at different stages in their development of DHTML;
from my own knowledge and what I've heard, DHTML in IE 4 is far more powerful
and versatile than NS 4's. I don't want to help spread any propaganda,
so I'll leave it at that.
-Tutorial
Introduction
-What is DHTML?
-DHTML
in NS- The <layer> tag
-DHTML
in IE 4
-Dynamic
Content
-Moving
elements around in the document
-Creating
cross-browser DHTML
-DHTML
resources
Discussion - Client Side Scripting
What is DHTML?