Can any one say what is the perpous of Fn key. I use sony viao. How the Fn is related to the F1,f2,.....F12 and Numlk key.

The Fn key changes the state of the keyboard so that the next key that is pressed while the Fn key is held down will generate a different scan code than it would without the Fn key. Look at it as an alternate Ctrl or Alt key.

How the Fn is related to the F1,f2,.....F12 and Numlk key.

it simply change the key from a F key to what ever symbol is on the lower part of the key

Usually press with other buttons as well to change the brightness and volume of laptops. Something like combo.

Yes. I have got it. But it is an extra feature of Sony Laptop. I do not find this in my Desktop.
Thanks.

no,its on all laptops/all i ever ownde or used anyway ,not on desktop computer