Look in the template directory of your UFM files and you'll see these files:

confirm.tpl.php
error_page.tpl.php
visitor.tpl.php
webmaster.tpl.php

These are simply html files with a placeholder in them:

{{content}}

This placeholder tells Ultimate Form Mail where to output the basic formatting for the html emails. In other words, Ultimate Form Mail strings together the form data, removes the fields that you tell it to remove, and then puts all of it right where the placeholder is located in your templates.


Advanced: Mail Merge

You also can put field name placeholders in your html templates. All you need to do is simply put the field names from your form in double braces like so:

{{first_name}}
{{email}}
{{message}}

If a placeholder is found in your template, but the form field value is empty (user didn't fill it in) then Ultimate Form Mail will remove the placeholder from your html template.