Multi-threading refers to an application with multiple threads running within a process, while multi-processing refers to an application organised across multiple OS-level processes.
A thread is a stream of instructions within a process. Each thread has its own instruction pointer, set of registers and stack memory. The virtual address space is process specific, or common to all threads within a process. So, data on the heap can be readily accessed by all threads, for good or ill.
Multi-threading is a more "light weight" form of concurrency: there is less context per thread than per process. As a result thread lifetime, context switching and synchronisation costs are lower. The shared address space (noted above) means data sharing requires no extra work.
Multi-processing has the opposite benefits. Since processes are insulated from each other by the OS, an error in one process cannot bring down another process. Contrast this with multi-threading, in which an error in one thread can bring down all the threads in the process. Further, individual processes may run as different users and have different permissions.
Difference between multitasking multiprogramming and multithreading?
Multiprogramming is a rudimentary form of parallel processing in which several programs are run at the same time on a uniprocessor.Since there is only one processor, there can be no true simultaneous execution of different programs. Instead, the operating system executes part of one program, then part of another, and so on. To the user it appears that all programs are executing at the same time.
Multitasking, in an operating system, is allowing a user to perform more than one computer task (such as the operation of an application program) at a time. The operating system is able to keep track of where you are in these tasks and go from one to the other without losing information
Multithreading is the ability of a program or an operating system process to manage its use by more than one user at a time and to even manage multiple requests by the same user without having to have multiple copies of the program running in the computer