The Export tool allows you to recreate your text and images and supporting meta data from your blog posts and pages in another site. This is useful, but it will not give you a backup of your theme, plugins and customizations. A full backup contains everything in your root (or public_html directory) - wp-content folder, wp-admin-folder, wp-confif.php file etc, as well as a backup of your database which contains all your blog/page content (as per the export tool).
It's important that you find a backup solution that works, and I recommend the BackUpWordpress plugin. It's easy to setup and you can download the backups for local storage on your machine. You can also backup your database through cPanel.