What is the difference between volt files and phtml files?

phtml is a template engine uses raw php,

example in phtml:

<?php echo $this->tag->form('/login'); ?>
    <?php echo $form->render('email'); ?>
<?php echo $tihs->tag->endForm(); ?>

volt is a template engine uses volt syntax (support also raw php):

example in volt:

{{ form('/login') }}
    {{ form.render('email') }}
{{ endform() }}

More about volt: https://docs.phalcon.io/en/latest/reference/volt.html