Nikhil Joshi wrote:

I looked at /etc/passwd file more carefully and found this

shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown halt:x:7:0:halt:/sbin:/sbin/halt

are these users ?

Anybody in /etc/passwd is logically a user, but in general ftp, mail, daemon, bin etc are system users, used by daemons(background processes, run ps aux)

I dont have any idea about purpose of halt and shutdown users. Try man halt or man shutdown.

Can I halt without logging in ?

You cant halt/reboot unless you login as root(superuser). You can however reboot by pressing ctrl-alt-del on local terminals OR switching the computer off ;-).

Amish.