In Balkan folkloric tradition, the hajduk (hajduci or haiduci in the plural) is a romanticised hero figure who steals from, and leads his fighters into battle against, the Ottoman oppressors. In short, they are seen as a local variety of Robin Hood and his merry men, who steal from the rich (which in the case of the hajduci happened to be also foreign occupants) and gives to the poor, while carrying a small guerilla war against an injust authority.
In reality, the hajduci of the 17th, 18th and 19th centuries were as much guerilla fighters against the Ottoman rule as they were bandits and highwaymen who preyed not only on Ottomans and their local representatives, but also on local merchants and travellers. However, most of the hajduci did follow a moral code which forbade robbing the poor and motiveless murder. Those who didn't, were no longer referred to as hajduci by the local population, but were called simply bandits.
The actual origin of the word hajduk is unclear. One theory is that hajduk was derived from the Turkish word haidud or haydut, which was originally used by the Ottomans to refer to Hungarian infantry soldiers. Another theory suggests that the word comes from the Hungarian hajtó (plural hajtók), meaning a (cattle) drover.