[NTLDR]

print_output() is used to compile the final page in any of the scripts (forum side). You'll notice that the lines with print output that are evaled in the forum side php files contain the template with the main skeleton HTML for the page. This function also calls process_replacement_vars() which as the name suggest does the replacement vars for the style you are on.

This is the only time you should echo directly out to the browser. fetch_template() does just as the name suggests, you're not supposed to echo its contents out untill the end of the page when you use print_output().