(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; /home/username. 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.
Root is the system administrator. It has permissions to access all commands and 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. A user has temporary access to the root account by prepending a command that requires root priveledges with the word sudo (SuperUser). Sudo asks for the user's password before executing the command.