Runlevels

Run level defines which services will start during booting. There are seven runlevels (0-6). A runlevel is a software configuration of the system that allows only a selected group of processes to exist. Init can run the system in one of eight runlevels. These runlevels are 0-6 and S or s. The system runs in only one of these runlevels at a time. Runlevels 0,1, and 6 are reserved. The runlevels are as follows:-

0(rc0.d)-halt (Don't set init default to this.)
1(rc1.d)-Single user mode.
2(rc2.d)-Multiuser without network file system(nfs)
3(rc3.d)-Multiuser mode (boot up in text mode.)
4(rc4.d)-Unused
5(rc5.d)-Multi user mode boot up in X-windows.
6(rc6.d)-Shutdown & Reboot(Don't set init default to this.)