  Stretched exponential relaxation of a quantity n versus time t according to n
= n_0 exp[-(lambda* t)^beta] is ubiquitous in many research fields, where
lambda* is a characteristic relaxation rate and the stretching exponent beta is
in the range 0 < beta < 1. Here we consider systems in which the stretched
exponential relaxation arises from the global relaxation of a system containing
independently exponentially relaxing species with a probability distribution
P(lambda/lambda*,beta) of relaxation rates lambda. We study the properties of
P(lambda/lambda*,beta) and their dependence on beta. Physical interpretations
of lambda* and beta, derived from consideration of P(lambda/lambda*,beta) and
its moments, are discussed.
