This article surveys the physics of systems proximate to Mott insulators, andpresents a classification using conventional and topological order parameters.This classification offers a valuable perspective on a variety of conductingcorrelated electron systems, from the cuprate superconductors to the heavyfermion compounds. Connections are drawn, and distinctions made, betweencollinear/non-collinear magnetic order, bond order, neutral spin 1/2excitations in insulators, electron Fermi surfaces which violate Luttinger'stheorem, fractionalization of the electron, and the fractionalization ofbosonic collective modes. Two distinct categories of Z_2 gauge theories areused to describe the interplay of these orders. Experimental implications forthe cuprates are briefly noted, but these appear in more detail in a companionreview article (S. Sachdev, cond-mat/0211005).