Nowadays, computer users (and mobile and tablet users too) use different applications at the same time when they work with their computers. They can be writing a document with a word processor while they're reading the news or posting in a social network and listening to music. They can do all these things at the same time because modern operating systems support multiprocessing.

They can execute different tasks at the same time. But inside an application, you can also do different things at the same time. For example, if you're working with your word processor, you can save the file while you're adding text with bold style. You can do this because the modern programming languages used to write those applications allow programmers to create multiple execution threads inside an application. Each execution thread executes a different task so you can...