Comment Code Inserting Blank Lines?

OK, so I’m new at trying to change code and it makes sense to me to add comments to my code so that I remember what the heck I’m doing, or what I did.

However, if I use code such as the following:
<!--Page Element Separation-->

On the page that I add this code to, it inserts a blank line, and if I look at the output code in Firebug, it shows <p></p> coding?

What am I doing wrong here?