Multithreaded Applications

Multithreading as a widespread programming and execution model allows multiple threads to exist within the context of a single process. These threads share the process' resources but are able to execute independently. The threaded programming model provides developers with a useful abstraction of concurrent execution. However, perhaps the most interesting application of the technology is when it is applied to a single process to enable parallel execution on a multiprocessor system.

That means that a single process can have many different "functions" executing concurrently, allowing the application to better use the available hardware (multiple cores/processors). Threads can communicate between them (they have shared memory), but its a hard problem to have every thread behave well with others when accesing shared objects/memory.

Threading allows an application to remain responsive, without the use of a catch all application loop, when doing lengthy operations.

For example, a non threaded copy program wouldn't allow you to do anything until the copy completes.

Threading helps with complex, lenghty, independent problems, but brings along a lot more complexity, that makes it hard even for seasoned developers.

There are lots of examples, as most application that need to interact with a user have a UI thread and a set of working threads. This is done to allow the UI to remain responsive while the application is busy doing some task.

For example, if you're tying a document in Word, there's a thread responding to your keyboard, there's a thread that's checking your spelling, there's one that's checking your grammar, there may be another thread saving a backup of your document in case the program crashes.

http://stackoverflow.com/questions/1313062/what-is-a-multithreaded-application