I have a very basic question about MPI. 

I have a computer with 8 processors (each with 8 cores).  What is the difference between if I run a program simply by "./program" and "mpirun -np 8 /path/to/program" ? In the first case does the program just use one processor out of the 8? If I want the program to use all the 8 processors at the same time, then I have to do with mpirun? 

Something fundamental is buggin me. 

Thank you,