1.Thread

• single threaded process - Single threaded programs have one path of execution, and multi-threaded programs have two or more paths of execution. Single threaded programs can perform only one task at a time, and have to finish each task in sequence before they can start another. For most programs, one thread of execution is all you need, but sometimes it makes sense to use multiple threads in a program to accomplish multiple simultaneous tasks.
• multi threaded process - This is multithreading, and generally occurs by time slicing (similar to time-division multiplexing) across the computer systems. However, in a single processor environment, the processor 'context switches' between different threads. In this case, the processing is not literally simultaneous, for the single processor is really doing only one thing at a time. This switching can happen so fast as to give the illusion of simultaneity to an end user.