Comment tags are used to insert comments in the HTML source code.
HTML Comment Tags
<!-- Write your comments here -->
Notice that there is an exclamation point (!) in the opening tag, but not in the closing tag.
Note: Comments are not displayed by the browser, but they can help document your HTML source code.
With comments you can place notifications and reminders in your HTML:
<!-- This is a comment --> <p>This is a paragraph.</p> <!-- Remember to add more information here -->
Comments are also great for debugging HTML, because you can comment out HTML lines of code, one at a time, to search for errors:
<!-- Do not display this at the moment <img border="0" src="pic_mountain.jpg" alt="Mountain"> -->
<!--[if IE 9]> .... some HTML here .... <![endif]-->
Conditional comments defines some HTML tags to be executed by Internet Explorer only.