Okay, programming languages are BASICLY separated into two types:low-level and high-level. In the beggining of programing, programmers wrote with their computers defined machine langauge (low-level). The bad thing about this was that they couldn't trade their code with anyone else and they had to learn how to program on each machine they encountered. To fix this, all the programmers began trying to make languages that were closer to English and that would work on any machine. So basically langauges over time have become higher and higher till now we have machine independent languages like C, C++, Fortran, Cobol, Pascal etc..
What happens with your compiler is that it transfers the high-level language to your computers machine language.
So in short, most of the code that you get online is in high-level form and it isn't compiled yet; when you compile it on your system, then well there you go.

BTW, don't quote me on being uber precise on this. Just a really rough guide.