MPI (or Message Passing Interface) is a parallel computing API used for distributed parallelism.

Essentially it's about

C programming language:

int MPI_Init(...);
int MPI_Finalize(...);
int MPI_Send (...);
int MPI_Recv (...);
int MPI_Bcast(...);
int MPI_Reduce(...);
int MPI_Group_size(...);
int MPI_Group_rank(...);
int MPI_Comm_rank (...);
int MPI_Comm_size (...);
int MPI_Type_commit(...);
int MPI_Type_contiguous(...);
int MPI_Type_free(...);

C++ programming language:

namespace MPI {
    class Comm {...};
    class Intracomm : public Comm {...};
    class Graphcomm : public Intracomm {...};
    class Cartcomm : public Intracomm {...};
    class Intercomm : public Comm {...};
    class Datatype {...};
    class Errhandler {...};
    class Exception {...};
    class Group {...};
    class Op {...};
    class Request {...};
    class Prequest : public Request {...};
    class Status {...};
};