Booting in Single User Mode in Linux

At the boot time, you usually see GRUB (GRand Unified Boot) Loader.

You select any one option of available OSs from the menu.

For example, if you have Windows and Linux.
You will get 2(or more) options to boot.

In order to boot into single user mode, you select desired
linux and press 'e' in grub menu, you will see new window
with something given below

root (hd0,0)
kernel /boot/vmlinuz-2.6.22.6 root=/dev/sda1 ro resume=/dev/sda3
initrd /boot/initrd.img-2.6.22.6
savedefault

Now, you append '1' or 'single' in the second line (kernel) as kernel
parameter. For example

kernel /boot/vmlinuz-2.6.22.6 root=/dev/sda1 ro resume=/dev/sda3 1

or

kernel /boot/vmlinuz-2.6.22.6 root=/dev/sda1 ro resume=/dev/sda3 single

Now, press 'b', to boot into single user mode. :)