The Ergative alignment, formally known as Absolutive-Ergative alignment, is a rather common grammatical case system. Absolutive-Ergative differentiates between the Agent, it being marked in the Ergative case, from the Subject, with the fusion of both seen in Nominative-Accusative systems. The Absolutive case marks the Object and Subject the same, often leaving them unmarked.
Distribution and Spread
The Absolutive-Ergative system is almost absent in Indo-European languages which are by far mostly Nominative-Accusative. In Europe, the only Ergative language is Basque. Ergative languages are spread thinly throughout the world, with certain pockets that contain a high concentration of Ergative languages. While Absolutive-Ergative languages cover an area from north Canada to Tibet and Iran, one of the main concentrations are in the Caucasus languages (think Georgian), Australian ones, and Eskimo-Aleut. Other Ergative languages exist, and Tibetan is a good example.