A computer can not understand your language. Neither it can understand C or any other computer language. So, how do we communicate with the computer? Its only true language is known as machine code. Unfortunately, it is very difficult to master. But, even for people who really know it, it is very unconfortable to use for practical purposes... This is what you do with the order g++ -o whatever whatever.cc: you compile your code, and the result --in machine code-- goes to the executable file.