A petabyte is 1 quadrillion bytes. A petabyte is big enough to hold about 250,000 standard definition movies. A related unit, the pebibyte, is 250 bytes, about 12.6% larger.
A gigabyte is 1 billion bytes. A typical standard definition movie is about 4 gigabytes in size. A related unit, the gibibyte, is 230 or 1,073,741,824 bytes.