IRQ is an abreviated computer term short for Interupt Request.

CPU does its computing cycles until a device like a printer or sound card or network adapter interupt the CPU so that the CPU gives its attention and cycles to what the device is asking for.

There are 16 IRQ Channels in every motherboard. Below is the standard layout of each IRQ channel:

IRQ 0 System Timer
IRQ 1 Keyboard
IRQ 2 Cascade to IRQ 9
IRQ 3 Com 2&4 (mouse)
IRQ 4 Com 1&3 (modem)
IRQ 5 LPT 2 (but usually Sound Card )
IRQ 6 Floppy drive
IRQ 7 LPT Port 1 (Printer)
IRQ 8 Real time clock
IRQ 9 VGA, NIC (redirect to IRQ 2)
IRQ 10 CD ROM Drive
IRQ 11 Open
IRQ 12 Open (Bus mouse port)
IRQ 13 Math co-processor
IRQ 14 IDE Controller (hard disk drive)
IRQ 15 PCI/ISA Bridge (motherboard)

see also ports & DMA.