Hi,

I am very new to ARM Processor.
What is the use of AT91 Library?

It contains some basic commands like setting up hardware, which is useful, if you dont like to write every single register by your own.

The libraries are a set of C source file sparing the time to write low level functions for using the peripherals.

But keep in mind that not only the C code will have to be added to your application. Have a look at the assembly startup files, thet may contain code that the C lib rely on to work.
A basic .s startup file provided by IAR or Keil, for example, may not work properly with the libraries.

I'm trying to use Atmel libs for SD card and USB. I could'nt manage to have it work without many adaptations to my application hardware and software.
(For the USB, I must say I'm still desperately trying to get a connection).

Denis