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The ''system calls'' are initiated by user-level programs, for opening files, starting other programs, etc. Each system call handler will have to check whether the arguments passed are valid, then perform the internal operation to complete the request.
 
Most user programs do not directly issue system calls (except for ASMasm programs, for instance), but instead use a ''standard library'' which does the ugly job of formatting arguments as required by the kernel and generating the system call. (For example, the C function ''fopen()'' eventually calls a kernel function that actually opens the file.)
 
The kernel usually defines a few ''abstractions'' like files, processes, sockets, directories, etc. which correspond to an internal state it remembers about last operations, so that a program may issue a session of operation more efficiently.
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