Not wanting to sound like an eco but those tags are from the W3C, they issue standards that programmers can use when creating sites, although what some of you think, these standards are important because you are building your site using todays browsers to test it, but tomorrows browsers may not understand them right because you didn't followed the standards.
I think that the most important thing to try to do is code your pages in XHTML (like HTML but you have to close all tags [ex. <br> would be <br />] and the document type is different) using semanthical coding and the correct tags for the content, for example, some of you must have done something similar to
(Imagine that this is going to be some news content)
HTML:
<table border="3" bgcolor="#CCCEEE" cellpadding="3">
<tr>
<td class="newsTitle">This is a title</td>
</tr>
<tr>
<td class="newsText">This is going to be a looong text...</td>
</tr>
<tr>
<td class="newsTitle">Oh look another news item</td>
</tr>
<tr>
<td class="newsText">And another looong text...</td>
</tr>
</table>
or worse...
HTML:
<table border="3" bgcolor="#CCCEEE" cellpadding="3">
<tr>
<td>
<font color="#0066CC" size="12pt">
<strong>
This is a title
</strong>
</font>
</td>
</tr>
<tr>
<td>
<font family="Times New Roman" color="#99AA45">
This is going to be a looong text...
</font>
</td>
</tr>
etc...
</table>
when this would be more correct
HTML:
<div id="news">
<h1>This is a title</h1>
<p>This is going to be a looong text...</p>
<h1>Oh look another news item</h1>
<p>And another looong text...</p>
</div>
And after this you would have a CSS styleshet determining the <h1> caracteristics (font, size, color etc) and <p> and a simple change in the CSS would apply to all pages, imagine that you wanted to change the color of the title, with the 1st example its not so hard, but using tables makes your code alot hard to understand, in the 2nd example you would had to go page by page changing that <font color=""> to whatever you wanted to change to, in the 3rd example the code is very clean and easy to understand not just for people but screen readers as well...
This is quite a long subject, and theres also downsides on the 3rd method, since some browsers *cough*IE*cough* don't displays all tags as they are specified in the W3C documents, well there is no browser that takes these documents by the letter, but they are improving and currently firefox, safari and opera stand in the frontline since they are updated regularly while IE (7 is coming) isn't since 2001.
Think about what W3C is doing and what it can do for you, there is alot to read about this on the web, google is powerfull, but don't throw standards to the bin, if you want to make a career as a webdeveloper/webdesigner or anything that requires (X)HTML coding, these are going to be important to you.