The term “bailiwick” has two senses. The first, older sense means an area under the supervision of a bailiff, a type of legal official whose role varies, depending on the nation. In the second sense, the term is used to describe an area of knowledge or skill, which is why you hear statements like “gardening is really Susan's bailiwick, not mine.” A few modern bailiwicks still exist, most notably in the Channel Islands of the United Kingdom.
The term is derived from the French bailie, a court official who was appointed to supervise an administrative district which was known as a bailliage or sénéchal, depending on the region. These officials were expected to oversee the justice system, collect taxes, and perform other administrative tasks. In England, the concept of a bailie was adopted, and these men became known as bailiffs; a number of Crown officials were called bailiffs, including sheriffs, tax collectors, and mayors.
By 1460, people were referring to the area under the supervision of a bailiff as a bailiwick, forming a compound from the words “bailiff” and “wik,” which meant “village.” Incidentally, “wik” is also the root in words like Greenwich, or “green village.” These administration districts were usually small, to allow the bailiff to keep an eye on the activities of all the citizens. The bailiff ensured that the authority of the Crown was upheld in his bailiwick, often with the assistance of an assortment of officials who performed various tasks.
In modern times, some people refer to a sheriff's administration area as a bailiwick, and in some regions a bailiff still sits in authority over a bailiwick. In most regions, however, the tasks of the bailiff have been broken up, and they are now supervised by a range of officials in a variety of districts. For example, taxes may be collected on a national basis, while law enforcement issues are handled by police forces.
In the second sense, a bailiwick can be an area of experience, influence, expertise, interest, or study. The term generally implies a broad and thorough scope, much like that of the bailiffs of old. In other words, if you casually play crosswords, you would not say that crosswords were your bailiwick, but if you completed the New York Times Crossword faithfully every day and studied the history and construction of crosswords, you could rightfully claim that crosswords were indeed your bailiwick.