Essentially, everything – it backups up your database and all WordPress content – which means your uploads, plugins, themes, and additional files created by plugins. The free version does not backup up WordPress itself, since you can always obtain this again from WordPress.Org. UpdraftPlus Premium has an option to include this too, for convenience – plus any files anywhere else on your server (i.e. non-WordPress) that you want to back up for good measure, too.
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