On: What is Differential Privacy?

[arnarbi]

> The output has some noise added to it.

You can also add noise to the input samples, so your database doesn't contain statistically significant information about any single individual. But aggregate queries will work as the noise evens out. See http://research.google.com/pubs/pub42852.html

Edit: Just saw that the linked article actually explains this in the last section as well.