The piece of Hardware that stores your programs and data "permanently" - ie after the power goes off. You probably know what a hard disk is.
The standard method of measuring large amounts of data (in the mainstream media) appears to be LibraryOfCongress x n.
To appreciate how far storage technology has come, look at the size of a 4 megabyte disk platter used at StanfordUniversity in the 1960s; there a $1 note in the lower-right corner to give you an idea of the platter's size. The black mark around it is from a disk head crash (!!).