There is no one language. It is usually multiple processes sharing data on a network.
OpenMPI is an example of an infrastructure used in clustered computers.
Folding@Home and
SETI@Home as well as
BOINC are examples of distributed, loosely coupled parallel computation models.
If you follow those links you will see that they all are associated with universities.
Before attempting parallel processing master single threaded programming. Then, you can use compiler PRAGMAs to parallelize you code to multiple threads on a single host. You could use OpenCL or CUDA to parallelize code onto a GPU. After you have exhausted the resources of a single host, then add the complexity of multiple hosts.