Hook – action and filter

I’m studying the code of wordpress, but I still can not understand how the hook works.
I understand the difference between “actions” and “filters”, but sometimes I find it difficult.
For example, the hook “user_edit_form_tag”. This hook is called one times in the file at line 192 in user_edit.php with
do_action (‘user_edit_form_tag’).
This means that there should be a function that is called user_edit_form_tag(), but it’s not, and I don’t understand what code is produced.
Someone I can do to help?
Exists a function for each hook type “action”?
Thanks.