<small>
For de-emphasizing inline or blocks of text, use the small tag.
This line of text is meant to be treated as fine print.
<em>
For emphasising a snippet of text.
The following snippet of text is emphasised.
<strong>
For strongly emphasising a snippet of text.
The following snippet of text is strongly emphasised.
<u>
For underlining a snippet of text.
The following snippet of text is underlined for emphasis.
<mark>
For marking a snippet of text such as a search term.
The following snippet of text is marked as found.
Feel free to also use <b> and <i>
In HTML5 <b> is meant to highlight words or phrases without conveying additional importance while <i> is mostly for voice, technical terms, etc.