su

Switch to superuser (root)

$ su

$ su [username]

$ su - [username]

$ su - [username] -c "[command]"

$ su -s /[path/to/shell] [username]

su [options] [-] [user]

su (substitute user) allows running a shell or command as a different user. Without a username, it switches to the root user by default.
The difference between su and su - is that the latter provides a full login environment, resetting environment variables and changing to the target user's home directory.

-, -l, --login

Provide a login shell environment

Pass a single command to the shell

Use the specified shell instead of the default

Preserve the entire environment

Preserve environment (same as -p)

Specify the primary group

Requires the target user's password (or root privileges). On many modern systems, sudo is preferred for running commands as root. Using su - is recommended over plain su to get a clean environment.

sudo(8), login(1), runuser(1)