In an ErgativeLanguage, the subject of an intransitive verb and the object of a transitive verb are considered to be the same case and are marked in the same way.

Indo-European languages, including English, are NominativeAccusative languages. 

Consider:

   The man opened the door.

   The door opened.

In English ''man'' is Nominative, whereas ''door'' is nominative in the second sentence and Accusative in the first.

In an ErgativeLanguage, ''door'' has the same case (Absolutive) in both sentences, ''man'' in the first sentence is Ergative.

It takes a lot of brain-warping to switch from one system to the other.

Basque, Georgian, and the Yucatec (Mayan) languages are ergative.

It has been suggested that IndoEuropeanLanguage was originally ergative.