Entropy and information divergence

The Kullback–Leibler (KL) divergence is a non-symmetric measure of the difference between two probability distributions P and Q:

KL divergence: $D_{KL}(P || Q) = \Sigma_{i}P(O_{i}) log \frac{P(O_{i}}{P(Q_{i})}$

In this piece [page 133] a neat example is given, showing how this measure can be used to quantify the `truthlikeness’ of a probability measure.