It is code which is executed when an interrupt occurs.

Interrupts can range from you typing ctrl-c, to the tick of your computer's clock, to reaching the end of a file, to opening a port, to the click of a mouse, etc, etc. Any interrupt sends a signal that gets the attention of the CPU. Hardware interrupts are generated when a key is pressed or when the mouse is moved. Software interrupts are generated by a program requiring disk input or output.

Interrupt handlers are written and registered with your computer so that it knows to execute them upon specific interrupts. They require special care when writing them. If you were to have an error in an interrupt handler that fired off its own interrupt, you would crash your computer. Usually when writing an interrupt handler, disable that interrupt until the handler code is done.     Answered by: richa