A petabyte is a collection of about 1 quadrillion bytes, which is around 1,000 terabytes.
In the metric system, peta is a prefix for 1 quadrillion, or 1015. So in some cases, a petabyte is defined as 1 quadrillion, or 1,000,000,000,000,000 bytes.
But because computers work in binary, computer scientists usually define a petabyte as 1,125,899,906,842,624 (10245 or 250) bytes. This works out to 1,024 terabytes.
The abbreviation for petabytes is PB in both the metric system (1,000,000,000,000,000 bytes) and the binary system (1,125,899,906,842,624 bytes).
Some groups have proposed the name pebibyte for a unit of 1,125,899,906,842,624 bytes to prevent confusion. The abbreviation for pebibyte is PiB.