what is a computer ? what is an operating system, a word definition?

what is computer

A computer is an electronic device that stores, processes, inputs, and outputs data to produce a result  

what is an operating system, a word definition?

It is an interface between the user and computer or hardware and software.

Operating systems can be classified as follows:

1.single-user singletasking

2. single-user multiutasking

3.multi-user singletasking

4.multi-user multitasking
multi-user : Allows two or more users to run programs at the same time. Some operating systems permit hundreds or even thousands of concurrent users.

multiprocessing : Supports running a program on more than one CPU.
multitasking : Allows more than one program to run concurrently.
multithreading : Allows different parts of a single program to run concurrently.
real time: Responds to input instantly. General-purpose operating systems, such as DOS and UNIX, are not real-time.

Common Operating SystemsThe primary operating systems in use are Windows (Windows 98, XP, Vista), Macintosh OS X, the many versions of Linux and Unix, i5/OS (IBM iSeries) and z/OS (IBM zSeries mainframes). DOS is still used for some applications, and there are many other special-purpose operating systems

What is an external / internal command?
Question:

What is an external / internal command?
Answer:

In MS-DOS there are two types of commands. 

An Internal command, which is a command embedded into the command.com file, and an external command, which is not embedded into command.com and therefore requires a separate file to be used.

For example, if your computer does not have fdisk.exe and you try using the fdisk command, you would receive an error "Bad command or file name." Fdisk is an external command that will only work if fdisk.exe, or in some cases, fdisk.com, is present.