<p>As the title describes, what are the different doctypes available and what do they mean? I notice that the layout looks a little different in IE7 when I switch from </p>

<pre><code>&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" &gt;
</code></pre>

<p>to</p>

<pre><code>&lt;!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"&gt;
</code></pre>

<p>Are there any others and what are the effects or ramifications?</p>

<p>Thanks!</p>

Here is the official explanation of the various DTD's from the W3C:
http://www.w3.org/QA/2002/04/valid-dtd-list.html
You might also find the following beneficial:
http://www.freedivs.com/tutorials/Choosing%20a%20DOCTYPE/