If the formatting (white space, new lines, indentation) of the code matters, use the pre element in combination with the code element:
<pre>
<code>
x = 42
if x == 42:
print "x is ... 42"
</code>
</pre>
You still have to escape characters with special meaning in HTML (like < with <), so for displaying a block of HTML code (<p>This is a paragraph.</p>), it could look like this:
<pre>
<code>
<p>This is a paragraph.</p>
</code>
</pre>
If a sentence contains computer code (for example, the name of an HTML element), use the code element to mark it up:
<p>The <code>a</code> element creates a hyperlink.</p>