Most modern operating systems have the ability to run more than one program at once. The user is lead to believe that the computer is doing many things at once such as play music, edit a document and perform a virus scan. The truth is much different. Computers are serial machines and by definition can only do one thing at once. If a computer has two cores then they can run two things at once. So even a quad core computer can only do four things at once. Essentially the user is fooled into believing that things are happening simultaneously.

The above diagram shows the idea that processes are swapped in and out of the CPU. Each process is swapped in and out so fast that the user is unaware that this is occurring. To the user it as if each processes is running at the same time. This illusion can easily be shattered if a process takes a lot of processing time or crashes. The user will then see the system hang.