  We characterize the class of homotopy pull-back squares by means of
elementary closure properties. The so called Puppe theorem which identifies the
homotopy fiber of certain maps constructed as homotopy colimits is a
straightforward consequence. Likewise we characterize the class of squares
which are homotopy pull-backs "up to Bousfield localization". This yields a
generalization of Puppe's theorem which allows to identify the homotopy type of
the localized homotopy fiber. When the localization functor is homological
localization this is one of the key ingredients in the group completion
theorem.
