there is a key in some computers : turbo key what is the use of this key

The "Turbo" key is essentially what many laptops these days have that's a "Fn" (Function) key. You typically use it in conjunction with other key to make use of some sort of shortcut. On desktop computer keyboards however this usually requires an associated driver (Although I guess some keyboards may have a few options of this kind working natively). Theoretically, you'd install a driver to have access to all those Turbo shortcuts that you could associate. For example, Turbo + M and it'll load your eMail client. Or Turbo + F and it'd load Firefox, stuff like so. In no way this is related to those old machines that had a "Turbo button" on the casing that worked with the CPU's clock speed.

hey i don't have any turbo key in my keyboard.. i heared it first time..so i have no more idea about it.. please tell me about this key..and it's functionality.. if you know..

If I am not mistaken the turbo key is redundant in many parts of the world. I have not seen it being used for a long time. Certain computers had a turbo key on the casing. I was made to understand that pressing it would speed up the CPU. This was at a time when CPUs were slow. Turbo capable computers had an LED display, showing the speed of the CPU. Somebody please correct me if my memory is faulty.