An example of a system call is open(). A process calls open() to open a file. The open function that gets linked to the program does reside in a library. But it very quickly hands the request of to the kernel. The kernel will open the file if it can. And either a file descriptor or an error message are returned to the user.

On the other hand, fopen() is the kind of function the folks call a library function. It will open a file, but it is not a system call. It must call open() to get the job done. A library function usually sits on top of coreesponding system calls. Or a library might be, say, math functions that can be implemented without a system call.

The distinction between system calls and functions gets a little blurred every now and then. getpid() is a system call that returns the process id of the caller. HP figured out a clever way to get the pid without invoking the kernel. HP now calls these "lightweight system calls". But they aren't exactly ssytem calls any more. Most versions of unix will claim that there are several system calls to exec another program: execl(), execle(), execlp(), execv(), execve(), and execvp(). But generally, only execve() actually exists and the other "system calls" invoke it.

This is why Posix lumps system call and functions together and simply calls them "interfaces".