Command

A command is a small program that does a specific task.
Commands are used to do many tasks such as :
1. copying programs
2. printing files
3. deleting files
4. running programs etc.
While using MS-DOS, you can consider a command as a tool you use to talk to MS-DOS. 

Device
To enable the computer to handle data or convert data into information, the data must be made available to the computer in the machine readable format, which is done by the device. Similarly the processed data is converted back into a form which the user can understand , this is also done by some other type of device.
Those devices which convert the raw data to be processed into a machine readable or computer understandable format is named as Input devices.
Examples : keyboard, mouse, joystick etc.
Those devices which convert the processed data back into a user understandable form is named as Output devices. Both input and output devices act as human machine interface.

Error
An error refers to a discrepancy between a computed value and a correct value. Similarly, if you are using improper command or instruction rather than using correct one, it causes an error.

Error Message
An error message is a notice on a display screen (monitor) informing the user of an incorrect data entry or command or machine malfunction.
For example, if you are trying to boot your computer from a diskette and the system disk is not set to A: or if the lock is still open, the computer will give an error message.
Non-system disk or disk error
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