What is IRQ?? (Interupt ReQuest)

IRQ is short for Interupt ReQuest. It's a signal that hardware devices generate to interupt the current operation of the CPU. It allows the CPU to pause and decide what to do next. It allows the PC to be able to respond to addition of new hardware. 

Take for example, a PC has tasks queued up that would take an hour to process. If there are no interupts, the PC would have to finish its current tasks before it gets to the task that the hardware added and process it. That is going to be very inefficient. 

That's thats why interupts are important. They enable the processor give time to the hardware if its priority is higher than the current tasks its handling.

Think also of your mouse, if you have ever been on a slow PC you would realize that sometimes everything but the mouse can be hanging. 

Thats because the mouse hardware generates an IRQ and its got a higher priority the processor processes its requests before going back to the requests that caused it to hang.

REFERENCE
http://www.computerhope.com/jargon/i/irq.htm