Who command: what is S on slackware

The who utility by default displays information relating to the login. If you logged in an tty2, then who will say so.

Use the w command to show information about the users currently logged into the machine and on what terminal they are doing what on.

Update: "What is the exact difference between a 'terminal', a 'shell', a 'tty' and a 'console'?" describes some differences between a TTY and a PTY. :0 is the particular host display that you are connected to.