What exactly does UpdraftPlus back up?

The free version backs up all of the files in the wp_content directory (e.g. plugins, themes, other files) and the WordPress database.

UpdraftPlus Premium can back up more files:

• Your wp-config.php file.
• Customisations made to WordPress core.
• Custom directories outside of the normal WordPress structure.

UpdraftPlus Premium can also back up more tables and external databases:

• Tables that are stored in the same MySQL database that don’t have the wp_ prefix. This is useful if you have MySQL applications integrated into the same site and need a backup solution that will work for them all.
• External MySQL databases. If you have other PHP/MySQL applications that aren’t related to any of your WordPress sites, you can back them up too with UpdraftPlus Premium.