system calls are provided by the system and are executed in
the system kernel. They are entry points into the kernel and
are therefore NOT linked into your program. These are not
portable calls.
·   Library calls include the ANSI C standard library and
are therefore portable. These functions are linked into your
program.
It is worth noting that, because system calls are part of
the O/S. The program has to make a context switch to the
kernel when they are called and because of this, they have a
high startup overhead. The upside is that the time executing
these routines is assigned to the OS and not the user program.

system call can be called from either kernel space or user 
space(in this case include switch to privilage mode) and 
executed in kernel space while library called only from 
user programm and may call systemcall to perform it is 
function like "printf" that need "write" system call while 
other not need kernel at all like sin(),cos().....

systemcall examples : write,read,close,mmap,open,.......
library examples: fopen,fclose,printf,scanf,fscanf,.....