  In this paper we study the correlations that arise when two separated parties
perform measurements on systems they hold locally. We restrict ourselves to
those correlations with which arbitrarily fast transmission of information is
impossible. These correlations are called nonsignaling. We allow the
measurements to be chosen from sets of an arbitrary size, but promise that each
measurement has only two possible outcomes. We find the structure of this
convex set of nonsignaling correlations by characterizing its extreme points.
Taking an information-theoretic view, we prove that all of these extreme
correlations are interconvertible. This suggests that the simplest extremal
nonlocal distribution (called a PR box) might be the basic unit of nonlocality.
We also show that this unit of nonlocality is sufficient to simulate all
quantum states when measured with two outcome measurements.
