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System call is nothing but OS call. To raise any service of kernel, then u need this system call. that is why sysstem calls are different from OS to OS.

Library calls are Language specific.
APIs calls are OS specific.
System calls are kernel specific calls.
Unless specific for each not changed, calls names will be same for each environment.

The above answer is not a convincing one.
System calls look like simple function calls. But, they call routines which run in the kernel space ( called Interrupt Service Routines ) and the functionality provided would be mostly related to hardware. So we can think system calls are a way in which OS provides way to talk to hardware.

The above answer is not a convincing one.
System calls look like simple function calls. But, they call routines which run in the kernel space ( called Interrupt Service Routines ) and the functionality provided would be mostly related to hardware. So we can think system calls are a way in which OS provides way to talk to hardware.

System call is interface between kernel level and user level i.e. it offers kernel services to user level

System calls -> User space applications request for kernel services.
Whenever there is a system call like open, read, close etc, with the help of system libraries the user application jumps to fixed location in kernel space by raising an interrupt (0x80 architecture dependent), where it can find what system call it is and etc