A system call is the mechanism used by an application
program to request service from the operating system.
  
  On Unix-based and POSIX-based systems, popular system
calls are open, read, write, close, wait, exec, fork, exit,
and kill. Many of today's operating systems have hundreds of
system calls. For example, Linux has 319 different system
calls. FreeBSD has about the same (almost 330).

Tools such as strace and truss report the system calls made
by a running process.

A system call is a request for the operating system to do 
something on behalf of the user's program.The system calls 
are functions used in the kernel itself.system calls are 
used to manage the file system, control processes and to 
provide interprocess communication.