Happy is the man that findeth wisdom, and the man that getteth understanding.
Proverbs 3:13 (KJV)
You (and anyone else who uses your computer) are a user. By default, a user’s files are only accessible by that user. Different users have different access to files, applications, and directories (folders). A user account is the sum total of the applications and files used to create the environment a user logs into.
A username identifies your user account to yourself (and to other users). It’s the name you log in with and the directory you own. Your username is a member of certain groups. The groups to which your username belongs tells the system what permissions you have with what (kinds of) files.
Your Home is at /home/username and it’s yours. Home contains your Desktop (customizable by you), your personal files (documents, music, pictures, videos, downloads) and your application settings (emails, address book, bookmarks). Each user on a system has their own Home.
Root is the administrator. It has permissions to access all commands and all files on the system. Root has the ability to modify everything in the system and control permissions for any user; including itself. To protect the system from potentially harmful commands, the root account is locked.
SuperUser A user in the sudo (SuperUserDO) group has temporary access to the root account by prepending a command that requires root privileges with the word sudo. The user is then asked for their sudo password (which is hidden when input) before the system will execute the command.
More Information root Definition at the Linux Information Project. RootSudo and HomeFolder on the Ubuntu Community Help Wiki. Linux File Permissions and Ownership Explained with Examples at It’s FOSS. How To Add User To Sudoers & Add User To Sudo Group on Ubuntu at phoenixNAP.