An unergative verb is an intransitive verb distinguished semantically by having an agent subject. For example, in English, run, talk and resign are unergative verbs (while fall and die are unaccusative).
In ergative–absolutive languages, the ergative case, which marks the subject of transitive verbs, usually represents the volitional role (i.e., the agent). But for unergative verbs, the subject is volitional and yet is not marked by the ergative.
Some languages treat unergative verbs distinctly from other intransitives in morphosyntactical terms. For example, in some Romance languages these verbs use different auxiliaries when forming the compound tenses. See the article on unaccusative verbs for details.
Besides the above, unergative verbs differ from unaccusative verbs in the fact that, in some languages, they can be passivized to a limited extent.
In Dutch for example, unergatives take hebben (to have) in the perfect tenses:
In such cases a transition to an impersonal passive construction is possible using the adverb er (that functions as a dummy subject) and the passive auxiliary worden
By contrast, ergative verbs take zijn (to be) in the perfect tenses.
In this case no passive construction with worden is possible. In other words, unergatives are truly intransitive, ergatives are not.