A. An Interrupt allows the piece of hardware to get the CPU's attention. For something like a Network card this is important as the card has limited buffer space so unless the CPU does not move the data out of the buffer it will get lost. Below is a table of the common IRQ uses.

IRQ Level Common Use Comments
0 Timer Hard-wired on motherboard
1 Keyboard Hard-wired on motherboard
2 Cascade from IRQ 9 May be available depending on Motherboard
3 COM2 or COM4  
4 COM1 or COM3  
5 LPT2 This is usually free as not many people have 2 parallel ports. Sound blaster cards usually use this.
6 Floppy disk controller  
7 LPT1 Sound blaster cards can use this
8 Real-time clock Hard-wired on motherboard
9 Cascade to IRQ 2 Wired directly to 2, sometimes tell software 9 when mean 2
10 Unused This is usually used by Network cards, many of them not allowing it to be changed
11 Unused Usually used by SCSI controllers
12 PS/2, Bus mouse If you are not using a PS/2 or bus mouse this can usually be used by another device
13 Math Coprocessor Used to signal errors
14 Hard disk controller If you are not using an IDE hard disk you may use this for another device
15 Some computers use this for the secondary IDE controller If you do not use the secondary IDE controller you may use this for another device

Note about attempting to free IRQ's used by unused motherboard devices: if your BIOS lets you disable the device manually and doesn't get reset by any Plug-and-Play software you have (for instance, Windows 95), you are probably okay. Otherwise, you'll just have to experiment to determine whether you can really use the IRQ occupied by the unused motherboard device.