個人的なファイル (ドキュメント、音楽、写真、ビデオ)

These are usually stored in your home folder (/home/your_name). They could be in subfolders such as Desktop, Documents, Pictures, Music and Videos.

If your backup medium has sufficient space (if it is an external hard disk, for example), consider backing up the entire Home folder. You can find out how much disk space your Home folder takes up by using the Disk Usage Analyzer.

隠しファイル

Any file or folder name that starts with a period (.) is hidden by default. To view hidden files, press the menu button in the top-right corner of the window of Files and press Show Hidden Files, or press Ctrl+H. You can copy these to a backup location like any other file.

個人的な設定 (デスクトップの設定やテーマ、ソフトウェア設定)

たいていのアプリケーションは、ホームフォルダー配下の隠しフォルダーに設定を保存します (隠しファイルについては上記を参照してください)。

Most of your application settings will be stored in the hidden folders .config and .local in your Home folder.

システム全体の設定

Settings for important parts of the system are not stored in your Home folder. There are a number of locations that they could be stored, but most are stored in the /etc folder. In general, you will not need to back up these files on a home computer. If you are running a server, however, you should back up the files for the services that it is running.