A petabyte (derived from the SI prefix peta- ) is a unit of information equal to one quadrillion (short scale) bytes, or 1000 terabytes. The unit symbol for the petabyte is PB. The prefix peta (P) indicates the fifth power to 1000:

• 1 PB = 1000000000000000B = 1000⁵ B = 10¹⁵ B = 1 million gigabytes = 1 thousand terabytes

But, in traditional binary usage, there are 1 125 899 906 842 624 bytes in 1 petabyte.
The pebibyte (PiB), using a binary prefix, is used for the corresponding power of 1024, which is more than 12% greater.
The unit petabyte is sometimes used for the corresponding power of 1024, following traditional designations of computer storage size, a practice superseded by modern conventions.
#from http://en.wikipedia.org