When the CPU needs data from the disk, it sends a signal to the disk drive telling it to locate the data and get it ready. Then, instead of just waiting the long and unpredicatalble amount of time the disk drive will take to do this, the CPU goes on with some other task. The interrupt handler can then read the requested data. Now, you might have noticed that all this only makes sense if the CPU actually has several tasks to perform. If it has nothing better to do, it might as well spend its time polling for input or waiting for disk drive operations to complete.
All modern computers use multitasking to perform several tasks at once.
Some computers can be used by several people at once. Since the CPU is so fast, it can quickly switch its attention from one user to another, devoting a fraction of a second to each user in turn. This application of multitasking is called timesharing . But even modern personal computers with a single user use multitasking. For example, the user might be typing a paper while a clock is continuously displaying the time and a file is being downloaded over the network.