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After the GRUB hands off the control of the system to the OS kernel.
The OS kernel starts the First process i.e with PID 0 "sched" which
short for schedular
sched is responsible for CPU time slicing, executing, and termination of
other processes.
Sched immediately  starts INIT Process
INIT is mainly responsible for starting the services in the specified runlevels
#which init
points to /usr/sbin/init
init reads the configuration file /etc/inittab to check for the RUN Level to Start in.

Various RUN Levels are as follows
RUN LEVEL      0   Shut down
RUN LEVEL      1   Single User -No Network Services
RUN LEVEL      2   Multi User  -Without NFS
RUN LEVEL      3   Multi User  -With NFS
RUN LEVEL      4   Customizable
RUN LEVEL      5   Multi User
RUN LEVEL      6   Reboots the system
who -r   = Show us the current RUN level