Glossary
Root
The administrator account with unrestricted privileges.
Root (UID 0) is the superuser: it bypasses permission checks and can do anything on the machine. Modern practice is not to log in as root but to stay yourself and borrow the privilege for individual commands with sudo, which authenticates you, applies a policy, and logs the act.
The word is overloaded: "root" is also the top of the filesystem tree (/) and the home directory /root. Context disambiguates, but the three are unrelated concepts.
See it in practice
$ sudo -lLists exactly what you are allowed to run as root on this machine. The first thing to check on an unfamiliar server.
Where you'll meet it
Temporary privilege, audited — and why sudo won everywhere.
chownAssign files to users and groups — the other half of Linux permissions.
File permissionsWho may read, write and execute — nine bits per file.
Filesystem hierarchyThe standard layout of directories under /.