As I understand it, the practice of distinguishing between agent (ergative) and non-agent (absolutive), rather than between nonpatient (nominative) and patient (accusative) as is common in Indo-European languages.
For a 'very' brief overview of different case-marking systems, see http://www.xibalba.demon.co.uk/jbr/ranto/r.html — part of a rant against Esperanto, but you can ignore that here; this appendix focuses only on cases. Ergative languages use approach (E) in his list.
How do we express this in Lojban? gausmi?