[wordpress] How to get the category title in a post in Wordpress?

Say I have a post called Hello World in Wordpress and I'm directly viewing this page, how would I go about finding the category of "Hello World" and displaying it?

You can use

<?php the_category(', '); ?>

which would output them in a comma separated list.

You can also do the same for tags as well:

<?php the_tags('<em>:</em>', ', ', ''); ?>