In single user mode only the system administrator can log in the account. Some of the administrative operations can only be done in single user mode. For example:
i) Unmounting a user partition.
ii) Fsck command on user partition whose file system is corrupted.
It is simple to start with Single User mode in Linux. All you need to do is to make small changes in the boot loader. Most of the distributions now use Grub as a boot loader / manager. This is how you edit grub to start a Single User mode:
a) Select Linux and hit Enter when Grub appears with OS choices menu .
b) After that scroll down to the line which starts with word 'Kernel' and then again hit Enter to edit it. Go the the end of the line, add a space and type in 'single'.
c) Now press Escape button in order to get to the main boot options screen, and then boot into Linux. Now a command prompt will appear instead of getting the GUI where you can carry out system maintenance tasks. You can change the password here as well whether you have been asked for root password or you have been given access to the root commands.
If you use LILO to boot Linux, just type in "linux single" at the LILO command prompt. Now you can change the root password with the following command when you are in Single User mode:
$ passwd root
After this you can reboot the system and go into GUI and can change all the users password as well. This is how you can get back into the Linux system after you lose your root password.
Any one can set his own password without knowing his original one by using the above steps. The idea of root is basically to prevent users from accidentally deleting other's data or modifying system files that are critical. If you enter into Single User mode then you must know that not to make careless mistakes.The server on which root would operate would also be physically protected. You can set Linux to prevent Single User mode. Hence emergency tasks need to be carried out using boot disk and logging in with the root password.