That "club card" at the grocery store let the store know the kinds of things you are likely to buy, so it can offer you a coupon for a future purchase.
And that's just one example of the ways in which data about us can predict future behavior.
We put a whole lot of data out there through computer and social networks, and it can be and is used to make predictions.
That has its uses, but can be scary as well.
Patrick Tucker of "The Futurist" magazine looks at both uses in his book The Naked Future.