For a computer at home, not connected to the Internet, knowing who is using your computer is usually pretty simple. You are. But UNIX allows us to log into our computer over a variety of connections: from the console, over a LAN, .... Most versions of Linux are set up to actually have many ``virtual'' text consoles (6 usually) and even graphical (X11) consoles. It is often handy to just start another login or console, instead of dropping what we were doing to start something else. So, even at home knowing who is logged into your computer can be handy, it might be you from a variety of different places.
There are 2 common commands for displaying who is logged in: who and w. They display some common information, and also some stuff that is different from each other.