OpenMP (Open Multiprocessing) is an API that supports multi-platform shared memory multiprocessing programming in C, C++, and Fortran, on most processor architectures and operating systems, including Solaris, AIX, HP-UX, GNU/Linux, Mac OS X, and Windows platforms. It consists of a set of compiler directives, library routines, and environment variables that influence run-time behavior.
OpenMP is managed by the nonprofit technology consortium OpenMP Architecture Review Board (or OpenMP ARB), jointly defined by a group of major computer hardware and software vendors, like AMD, IBM, Intel, Cray, HP, Fujitsu, Nvidia, NEC, Microsoft, Texas Instruments, Oracle Corporation, and more.
OpenMP uses a portable, scalable model that gives programmers a simple and flexible interface for developing parallel applications for platforms ranging from the standard desktop computer to the supercomputer.
An application built with the hybrid model of parallel programming can run on a computer cluster using both OpenMP and Message Passing Interface (MPI), or more transparently through the use of OpenMP extensions for non-shared memory systems.
Read more about OpenMP: Introduction, History, The Core Elements, Sample Programs, Implementations, Pros and Cons, Performance Expectations, Thread Affinity, Benchmarks, Learning Resources Online