EAR (Enterprise application ARchive) is a file format used by Java EE for packaging one or more modules into a single archive so that the deployment of the various modules onto an application server happens simultaneously and coherently. It also contains XML files called deployment descriptors which describe how to deploy the modules.
We usually called the Nuxeo base archive as an EAR for historical reasons: it was mainly designed as an EAR for application servers (JBoss). But, that's no more the case. The current archive is specific to Nuxeo and reorganized while being deployed depending on the target server. So, from now on, that concept is named NXR.
See NXR definition.