When a program is ran with MPI all the processes are grouped
in what we call a communicator. You can see a communicator as a box 
grouping processes together, allowing them to communicate. 
Every communication is linked to a communicator, 
#allowing the communication to reach different processes. 
Communications can be either of two types :
    -Point-to-Point : Two processes in the same communicator are going to 
                      communicate.
    -Collective : All the processes in a communicator are going to communicate 
                    together.
The default communicator is called MPI_COMM_WORLD. 
It basically groups all the processes when the program started