Ergativity refers to intransitive subjects being marked like transitive objects, and transitive subjects being marked differently. (This is the inverse, in a sense, of nominative-accusative languages like English, etc.)
Read this link for more:
http://en.wikipedia.org/wiki/Ergative%E2%80%93absolutive_languageA "split" ergative(-accusative) language is one where sometimes it's ergative and other times it's accusative. This is actually extremely common: I think I heard once that there are no completely ergative languages.
The exceptions are usually in some part of the grammar such as:
1. Pronouns vs. full nouns (or certain persons, etc.) (This is found in Australia I believe)
2. Tenses (Hindi has an ergative past tense system but accusative otherwise)
3. Some languages may have a distinction between morphology and syntax, such as which one is the subject after a coordinated clause with 'and'-- "Bill saw John and then he fell down"-- who fell down?
(Note: you should verify this information well before trusting it-- I'm remembering this info from a while ago, but it's on the right track at least.)
