Words or phrases can be emphasised by using the <em> and <strong> elements. For example:

<p>I believe the Dave Raggett book is <em>really</em> good. 
Phone me <strong>after lunch</strong> please.
<em>Really urgent that I <strong>talk</strong> to you!</em></p>

which would appear as

I believe the Dave Raggett book is really good. Phone me after lunch please. Really urgent that I talk to you!

• <em> emphasises the text
• <strong> emphasises it even more
• <strong> inside <em> should emphasise it even more
• How is up to the browser unless a stylesheet is provided.
• <em> may be italic
• <strong> may be bold
• <em><strong> might be bold italic

And what browser I am using actually does.