<script> - This is used for client side coding languages, you write or include your JavaScript or whatever it is in there. So the browser knows its not HTML.
<a> - Is an anchor, when an href attribute is added like so:
<a href="location.html">Click here</a>[/hml]
Thenit now becomes a link ( i think href stands for hyper reference or something like that ), so in the example, if you click the link you will go to location.html
href - Basically just explained above. It points to a file or location, it can be used in other things, not just links. For example it can be used to link in a stylesheet like so:
[html]<link href="location.css" type="text/css" media="screen" rel="stylesheet" />
Definition list - This is just another type of list, you have <ul>'s ( unordered ), <ol>'s ( ordered ) - these are the 2 main lists, which form either bulleted or numbered. For a definition list you would have something like:
<dl>
<dt>Title</dt>
<dd>Description</dd>
<dt>Title Again</td>
<dd>Description Again</dd>
</dl>
As for <comment>, there is no such thing. To do a comment its like this:
<!-- The comment goes between these arrow things. -->