This is the basic who command with no command-line arguments. It shows the names of users that are currently logged in, and may also show the terminal they're logged in on, and the time they logged in.

who | more

In this example the output of the who command is piped into the more command. This is useful when there are a lot of users logged into your computer system, and part of the output of the who command scrolls off the screen. See the more command for more examples.

who -a

The -a argument lists all available output of the who command for each user.