question about OS??

Hello, I am kind of new to programming, and one of my courses requires that I study operating systems hence I have started reading up on it on my own...I am very confused on what system calls are?? I have looked it up in several books, which give a very detailed definition, and I couldn not find any internet sites that had a general overview of system calls, but I would really appreciate if someone could give a brief description of what a system call is??? thanks in advance