Comments in HTML or PHP pages (outside of the PHP code) look like this:

Comments in CSS files look like this:
/* comment here about a style */

Comments inside of PHP code look like this:

<!-- comment here about what is going on -->

< ?php the_excerpt(); // Show excerpt and not full post content ?>

Or like this:

< ?php /* This is my special hack. It’s so special it requires a comment that spans multiple lines! */ my_special_hack(); ?>