A thread is a dispatchable unit of executable code. The name comes from the concept of a “thread of execution.” In a thread-based multitasking environment, all processes have at least one thread, but they can have more. This means that a single program can perform two or more tasks concurrently. For instance, a text editor can be formatting text at the same time that it is printing, as long as these two actions are being performed by two separate threads. The differences between process-based and thread-based multitasking can be summarized like this: Process-based multitasking handles the concurrent execution of programs. Thread-based multitasking deals with the concurrent execution of pieces of the same program.

The terms multitasking and multiprocessing are often used interchangeably, although multiprocessing sometimes 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.

There are two basic types of multitasking: preemptive and cooperative. In preemptive multitasking, the operating system parcels out CPU time slices to each program. In cooperative multitasking, each program can control the CPU for as long as it needs it.

Multiprocessing can be said to be either asymmetric or symmetric. The term refers to how the operating system divides tasks between the processors in the system. Asymmetric multiprocessing designates some processors to perform system tasks only, and others to run applications only. This is a rigid design that results in lost performance during those times when the computer needs to run many system tasks and no user tasks, or vice versa. Symmetric multiprocessing, often abbreviated SMP, allows either system or user tasks to run on any processor, which is more flexible and therefore leads to better performance. SMP is what most multiprocessing PC motherboards use. 

In order to employ multiprocessing effectively, the computer system must have all of the following in place:

Motherboard Support: A motherboard capable of handling multiple processors.
Processor Support: Processors that are capable of being used in a multiprocessing system.
Operating System Support: An operating system that supports multiprocessing.

The first interactive user interfaces to computers were not graphical; they were text-and-keyboard oriented and usually consisted of commands you had to remember and computer responses that were infamously brief. The command interface of the DOS operating system is an example of the typical user-computer interface before GUIs arrived.

Today's major operating systems provide a graphical user interface. Applications typically use the elements of the GUI that come with the operating system and add their own graphical user interface elements and ideas.

Elements of a GUI include such things as: windows, pull-down menus, buttons, sliders, scroll bars, iconic images, wizards, the mouse, and no doubt many things that haven’t been invented yet.