Dear Reader,
Please let me know the meaning of the following code supplied by Atmel :

#define PIN_USART0_RXD {1 << 26, AT91C_BASE_PIOA, AT91C_ID_PIOA, PIO_PERIPH_A, PIO_DEFAULT}

In simple words what is the difference bwtween
a. #define(A,B)
b. #define{a,B}

Recently I bought an ARM 926 board and found the above mentioned code as trial software.

a) Passes parameters
b) Is a compound statement / array entry

It sets up a GPIO initialization structure for pin PIOA.26, to the "A" side peripheral mux. So the pin on the device is connected to the receive pin of the UART Zero.

Is this a 9263?

Yes,
It is for AT91SAM9263_P863. But I am still confusion how this structure is used. I went through the files but could not find where it is being used; not only PIN_USART0_RXD but I tried other macros like PIN_SPI0_MISO, PIN_SPI0_NPCS3.
It is going beyond my comprehension how the structure is used for configuring a pin. I worked earlier on simple processors like 8051 and AVR 8 bit so finding this complex macro and uses a bit difficult.
Any suggestion or link would be highly appreciated.

They are used to build pin tables for the PIO_Configure() subroutine. The include file you found them in defines the board characteristics, whether it is used or not will depend on if the particular pin/peripheral is needed. If you use a slightly different board you just need to update the include file, this is how Atmel deals with the 9260-EK, 9263-EK, etc.

Here's how the text substitution macros work for the SDRAM pins

static const Pin pinsSdram[] = {PINS_SDRAM};

// Enable corresponding PIOs
PIO_Configure(pinsSdram, 1);

where board.h handles D16..D31 thusly..

/// List of all SDRAM pins definitions.
#define PINS_SDRAM {0xFFFF0000, AT91C_BASE_PIOD, AT91C_ID_PIOCDE, PIO_PERIPH_A, PIO_DEFAULT}

You could do this manually by programming the bits in the PIO controller "SFR", but that is prone to error, and you have a lot more pins. This method is cleaner and more efficient.