In computer science, a software can be seen as a stream with a start and exit point. Each software starts its life with the main() function in C/C++. This is the entry point of your program. Until this point, you are able to do whatever you want; including creating new routine streams, cloning the entire software, and starting another program. The common point with all these examples is that another stream is created and has its own life, but they are not equivalent.
This functionality is pretty simple. Calling fork() will duplicate your entire running process to a new one. The new process that is created is totally separated from its parent (new PID, new memory area as the exact copy of its parent), and will start just after the fork() call. The return value of the fork() function is the only difference between the two executions.
Following is an example of the fork() function:
int main()
{
int pid = fork();
if(pid == -1)
std::cerr<<...