The question as to how languages are alike and different and why is probably as old as speech itself. Some aspects of language are as universal as the human capacity to speak, while others vary, some more systematically than others. More than chance similarities among Indo-European languages have been explained on the basis of earlier periods of shared communities of speakers.
At least as interesting are systematic non-historical similarities. Typological structures recur across the world's languages, suggesting that the underlying properties of language are not yet fully understood. In the second half of the twentieth century work intensified, building on Edward Sapir's classification of languages based on morphological types: inflectional (e.g. older IE), analytic (e.g. Chinese, English), and agglutinating (e.g. Turkish). Joseph Greenberg's empirical investigations of implicational universals laid the basis for many studies of word order typology and formal variation within otherwise universal grammatical categories. A web page describing a conference held in his memory, Global Perspectives on Human Language (April 2002), includes a map locating the major language families of the world and abstracts of the papers presented in his honor. (A "Greenberg Conference" link will be provided on our Links page so long as the target page exists at Stanford; if that page disappears, our link will [soon thereafter] be deleted.)
Bernard Comrie's and Matthew Dryer's work catalogs language structures as they are found in a 200-language sample of non-Indo-European languages, and a world map locates them geographically. (A "World Atlas of Language Structures" link will be provided on our Links page so long as the target page exists at SUNY Buffalo; if that page disappears, our link will [soon thereafter] be deleted.)
Russian work on contentive typology has called attention to semantic factors affecting language type. Papers in volume 2 of the JIES Lehmann Festschrift (Journal of Indo-European Studies Monograph 31) presents a range of current typological approaches, with references. Brigitte Bauer's projects to index publications based on Indo-European typology (Language Systems, and Word Order) focused on correlates of alignment types.
Exploration of language classification issues continues, for example with the 2007 Workshop on Alternative Approaches to Language Classification at Stanford University. (A "Workshop on...Language Classification" link will be provided on our Links page so long as the target page exists at Penn State; if that page disappears, our link will [soon thereafter] be deleted.)