My friend Bradley Mazurek explains a petabyte:

petabyte:
2^50 = 1,125,899,906,842,624 bytes = 1024 terabytes or roughly 10^15 bytes

where a terabyte is:
2^40 = 1,099,511,627,776 bytes = 1024 gigabytes or roughly 10^12 bytes

So, a petabyte is well over 1,000,000 gigabytes.

In short, a petabyte is what you get when you multiply the number of Ritz crackers it'd take to reach Neptune by the amount of methane emitted annually by cows laid end to end.