In MPI the most essential operations are:
MPI_Sendfor sending a message
int MPI_Send (void* buf, int count, MPI_Datatype datatype, int dest, int tag, MPI_Comm comm)
MPI_Recvfor receiving a message
int MPI_Recv (void* buf, int count, MPI_Datatype datatype, int source, int tag, MPI_Comm comm, MPI_Status* status)
Although there a 100+ MPI functions defined in the standard (e.g. for non-blocking or collective communication, see the References for more details), you can write meaningful MPI application with less than 20 of those. Programs written with these functions have to be compiled with a specific compiler (options) and executed with a special startup program like detailed here.
Please check the more detailed tutorials in the References.