we have 2 modes in cpu .
kernel mode and  usermode.
kernel provides service to the user on request.
user executes the commands when kernel provides service to 
it.after execution completes kernel  looks after the system 
drivers (network drivers)
  user has no privelages  until kernel provides service to 
the  user.

actually the layers are 
outermost layer is user
inner layer is kernel
and inner most layer is hardware.

so when user sends any command it willnot go directly to 
hardware.first it will go and ask permission from kernel 
and if kernel permits then the command will reach hardware.

so unlike in other operating system the user cannot 
directly interact with hardware

kernel is the core of the operating system.It is interface 
between user system hardware.

Kernel of UNIX OS can be defined as a subsystem which is 
responsible for almost all tasks of an OS. Mainly some of 
the responsibilies of Kernel are Process dispatching, 
Process management, Memory management and so on.

Kernel is a heart of unix operating system,
it act as a interface between the computer hardware
and user. In unix all the commands are accepted by 
sting operation. command interpretor(shell interpretor) 
want to execute the command in computer hardware. here the 
kernel takes the commands by command interpretor(shell 
interpretor) and executes in computer hardware(where the 
unix is loaded)Finally it produce the result.