Debianadmin have a small overview article about runlevels – what are they and how to change them.
Read it here
Linux run levels are numbered 0 through 6. Run levels stop at six for practical and historical reasons, but it is entirely possible to have more if desired.
The following table summarizes the User Linux run levels:
* 0 System Halt
* 1 Single user
* 2 Full multi-user mode (Default)
* 3-5 Same as 2
* 6 System Reboot