A good list of these elements may be found here.
B (or bold) Element
In the past, this element simply indicated bold text. Today the element represents text that needs extra attention.
<b> and </b>
Strong Element
The strong element emphasizes text to show importance.
<strong> and </strong>
I Element
This element used to indicate italics; today it’s used to differentiate text.
<i> and </i>
Em Element
This element is used so stress the emphasis of contents.
<em> and </em>
Small Element
The small element represents small print -- related content.
<small> and </small>
S (or Strikeout) Element
This element represents content that is no longer accurate or relevant but has been left on the page.
<s> and </s>
Cite Element
The cite element indicates a title of a work. For example, this would be used to indicate a book or article title.
<cite> and </cite>
Q (or Quote) Element
The q element reprents a short quotation.
<q> and </q>
Dfn Element
The definition element represents a term being defined.
<dfn> and </dfn>
Abbr (Abbreviation) Element
The abbreviation element simply represents an abbreviation. When combined with a title attribute, the full expansion of the abbreviation can be displayed.
<abbr title="Hypertext Markup Language">HTML</abbr> is a good thing to know.
Data Element
This element is used in conjunction with microformats or microdata attributes to provide machine-readable content in addition to human-readable content. While beyond the scope of this document, do learn about this!
Time Element
The time element is used to represent a date or time. There are several valid formats.
The datetime attribute is associated with the time element; this attribute represents machine-readable values. Of course the value of this attribute should match that specified in the time element.
<time> and </time>
Code Element
This element is used to represent a fragment of computer code; the computer language is not specified. However it's possible to indicate the language if needed. I use this frequently when presenting demonstrations of HTML and CSS.
<code> and </code>
Pre (Preformatted) Element
This element is used to preserve existing format: while not semantic, the pre element is often used with the code element.
<pre> and </pre>
Var Element
The variable element represents -- a variable!
<var> and </var>
Samp (Sample) Element
The sample element simply presents sample output from a program or system.
<samp> and </samp>
Kbd Element
The keyboard element represents user input.
<kbd> and </kbd>
Sub Element
The Sub element represents a subscript.
<sub> and </sub>
Sup Element
The Sup element represents a superscript.
<sup> and </sup>
U Element
This element represents text that also has a non-textual meaning or annotation. Note that text contained within this element is often rendered with an underline--that is often confusing since hyperlinks are formatted in the same manner. In pracice this is rarely used.
<u> and </u>
Mark Element
The mark element essentially highlights text.
<mark> and </mark>
