Multitasking refers to the capability to perform more than one task at a time. Tasks in this context are usually programmes (or programs if you use the US spelling).
Multitasking should not be confused with multiprocessing because this usually implies that more than a single CPU is performing the task or tasks. In multitasking only a single CPU performs the tasks, switching between the tasks and giving the impression that the tasks are being performed simultaneously, which in effect they are, but by switching rapidly from one task to the other rather than actually performing them simultaneously.
Preemptive multitasking is where the operating system the CPU decides how to apportion time between the two (or more) tasks, whereas cooperative multitasking is where the programmes control the CPU, using up time until the task is executed.
The ability to execute more than one task at the same time, a task being a program. The terms multitasking and multiprocessing are often used interchangeably, although multiprocessing implies that more than one CPU is involved.
In multitasking, only one CPU is involved, but it switches from one program to another so quickly that it gives the appearance of executing all of the programs at the same time. OS/2, Windows 95, Windows NT, the Amiga operating system and UNIX are based on pre-emptive multitasking while Microsoft Windows 3.x and the MultiFinder (for Macintosh computers) are based upon cooperative multitasking.
There are other uses for the phrase multitasking though, and these relate to the ability of people to perform several tasks simultaneously. Some people are good at this, while others perform less well when they try. For example, if you try to drive while talking on your mobile phone you might find it sufficiently difficult that you crash, or say the wrong thing while talking because you are avoiding the crash.