The office of the Croatian bán however is older than the acquisition of the Croatian throne. It was there at the very beginning of their statehood. Croatia was formed by 11 tribes, settling in 11 counties after they migrated here. They pushed out some previous Slavs from power, but Avars also lived in their lands. The three counties they lived in gained a special status, and to lead and govern those areas the office of the bán was created. It was a military rank as well (as in those times frequently happened). Two explanations exist for the origin of the word, one of wich points to the the Slavic Pan as root, the other to the Turkic bayan meaning wealthy, rich (see the Turk khagan, called Bayan; also might be related to Hungarian bő which was also a rank, and has the meaning: lot, many, wealthy, etc...).
The status of the bán and how he was elevated to the rank changed during the centuries, sometimes the Croatian Sabor - which was essentially the parliament of the Croatian estates (even today they call their Parliament that) - elected him, sometimes the king handpicked him, sometimes one suggested the other picked from the suggestions, sometimes the Hungarian parliament decided in the question, sometimes the position left empty (during the Habsburg times). Sometimes there were two báns, one for Croatia-Dalmatia and one for Slavonia, a region bordergin Hungaria proper, which wasn't part of Croatia proper either, sometimes governed by the Hungarian prince when the king opted to make him a co-ruler.
The Ottoman Wars and the Ottoman conquest of central Hungary also effected Croatia. Some of the lands lost, from some of the rest, after the Habsburgs acquired the Crown, they organized a border guard region which wasn't under the control of the bán but the emperor's military council - this is the reason that sometimes they did not pick a new bán, although several laws were passed about the issue. Much later, way after the events of 1848-49 tbey gave back the Croatian civilian control over those areas. The last bán was Ivan Subasic who wore this ancient rank until effectively 1945.