Quote from: http://en.wikipedia.org/wiki/Bitcoin#Generation
Because block solutions arise out of an independent random process, block creation by the Bitcoin network can be described as a Poisson process. The protocol adjusts the problem difficulty so that the distribution mean is λ = 2016 blocks per two weeks, so there are roughly ten minutes between the creation of new blocks on average (the wait times between events in a Poisson process follow an exponential distribution). The difficulty updates happen every 2016 blocks. The difficulty is set to the value that would have most likely caused the prior 2016 blocks to take two weeks to complete, given the same computational effort (according to the timestamps recorded in the blocks).[25] All nodes perform and enforce the same difficulty calculation.
