Pilosa uses roaring bitmaps to store the data. Roaring bitmaps are compressed and performing bit operations on them is very efficient since they don't require uncompressing the whole bitmap to perform bit operations.
The Roaring bitmap paper is equally opaque in this regard. I easily understand everything about the bitmaps themselves: the compression and various operations. But then they say that they tested it on some Census1881 data, and I'm thinking, what, how? How do you grind census data into a bitmap? And is everything in the bitmap, including people's names and such, or does the bitmap refer to objects that are not in the bitmap?