Using system calls

Hi,
I'm new to UNIX system calls. Can someone share your knowledge as to how exactly system calls should be executed?

Can they be typed like commands such as mkdir on the terminal itself? Also, are there any websites which will show me an example of the output to expect when a system call like open() is executed?

Thanks.

System calls are subroutines or functions that can be invoked from a C program. The program that invokes one must test the reurn code to see if it worked. If not, the program may display an error. But what message, if any, will be displayed is up to the programmer who wrote the program.