post_form() function is used in bbPress to output the form which can be used variously - as a New Topic post and in Reply Post

Basically it calles the post-form.php file, where francly, evertyhing is done just well. You would only need to use CSS rules to make it better.

On the other hand, post_form() allows the use of additional arguments:

post_form() arguments:
h2 = empty;
last_page_only = true;

These are easy to use. For instance, the following code
post_form(\'h2=New Topic&last_page_only=FALSE\')
when used in forum.php will result in adding a New Topic post form labeled New Topic, which would be added on every page of the forum.php (as opposite to only the last page of the forum list)