comment_form
comment_form is an echo-only action that is fired within an entry's comment form. Its position within the form is not predictable. This action is primarily used for adding additional fields (hidden or visible) to the comment form. It takes one parameter... the post's ID.
Context:
File: wp-content/themes/{theme-name}/comments.php
<?php do_action('comment_form', $post->ID); ?>
This hook is theme-dependant which means that it is up to the author of each WordPress theme to include it. It may not be available on all themes, so you should take this into account when using it.
This hook is an action which means that it primarily acts as an event trigger, instead of a content filter. This is a semantic difference, but it will help you to remember what this hook does if you use it like this: add_action('comment_form', 'your_function');