http://mathoverflow.net/questions/2931/localizations-of-categories
http://nlab.mathforge.org/nlab/show/localization
http://www.ncatlab.org/nlab/show/simplicial+localization
Sullivan’s MIT notes (Homotopy folder). Excellent stuff on localizations, completions, profinite, p-local and rational homotopy types. p-adic homotopy theory. Relation to etale homotopy theory! Remark: The Sullivan conjecture is about the contractibility of the space of maps from the classifying space of a finite group to a finite-dimensional CW complex.
nLab page on Localization 3