An IRQ is an electrical signal generated by a device needing attention. Usually, it means that some event has occurred such as data being received and ready to use. The IRQ signal is sometimes connected directly to the processor or in some cases to a dedicated interrupt handling circuit. When the processor is told the interrupt has happened it calls the ISR to handle it.
The ISR is software that deals with the needs of the device that requested it. Usually, the device is read or some system action is taken to clear the condition that caused the IRQ then the interupt system is reset to make it ready for use again.
On most systems, the IRQ signal forces the processor to save it's current program counter so it knows where to return when the ISR is finished, then it jumps to a hard-wired address called the Interrupt Vector where the code for the ISR is placed. At the end of the ISR, the program counter is restored and the program carries on from where it was before the interrupt paused it.
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