A MultiThreaded application is one which has more that one thread of execution, potentially executing in parallel. Most modern GUI applications (and all JavaGUI programs) are MultiThreaded. Responsive GUIs are much easier to write with threads - just run separate threads to get actual work done and have one or more threads take care solely of the GUI. Without threads, you'd have to make sure to constantly check back whether the user has requested anything while you're in the middle of your heavy lifting.
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