Parallel Processing can be achieved in two different ways, multi-core and single process threaded architecture. In a Multi core environment independent operations are shared between multiple CPU cores so as to obtain considerable improvements in performance. A single core threaded setup uses a single process with multiple threads for each individual operations using thread context switching for switching between the execution of different threads. Threads are ideal where operations wait for longer times during I/O operations. When one Thread is blocked for I/O a context switch takes place to another thread. This results in collective improvement in performance.
A very simple example of using parallel processing(Threaded or Parallel Processes) is by the gem Parallel.
Note: Using Threads are dangerous and could result in actually slowing your execution if done wrongly be Warned.