An unergative verb is an intransitive verb that is characterized semantically by having a subject argument which is an agent that actively initiates the action expressed by the verb.
For example, in English, talk and resign in the sentence "You talk and you resign" are unergative verbs, since they are intransitive (one does not say "you talk someone") and "you" are the initiator or responsible for talking and resigning. 
But fall and die in the sentence "They fall and die" are unaccusative verbs, since usually they are not responsible for falling or dying but still the verb is intransitive, meaning it is comprehensively used without a direct object. (They cannot "fall something" or "die someone").
Some languages treat unergative verbs differently from other intransitives in morphosyntactic terms. For example, in some Romance languages, such verbs use different auxiliaries when in compound tenses.
Besides the above, unergative verbs differ from unaccusative verbs in that in some languages, they can occasionally use the passive voice.
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 by using the adverb er, which functions as a dummy subject and the passive auxiliary worden:
By contrast, Dutch ergative verbs take zijn ("to be") in the perfect tenses:
In that case, no passive construction with worden is possible. In other words, unergatives are truly intransitive, but ergatives are not.