Koreans usually give their surname (seong) together with a clan seat (bongwan) — as in "Gimhae Kim" or "Jeonju Lee." The surname marks patrilineal descent, while the clan seat denotes the region where that surname's founding ancestor is held to have settled. Even among Kims, the Gimhae Kim and the Gyeongju Kim are regarded as separate branches with different clan seats. A record of these surnames and seats through the generations is precisely the jokbo, the family genealogy.
The people who had no surname
Everyone has a surname now, but until the early Joseon period surnames belonged mainly to the ruling class and some free-born people. Many of the lower orders, the nobi above all, were often called by given name alone, without a surname. To have a surname and clan seat was in itself to show that one belonged to a proper family line, so surnames were deeply bound up with status.
How everyone came to have a surname
As the class system shook in the late Joseon period, those who had raised their standing increasingly acquired a surname and clan seat and entered a genealogy. Then, with the enforcement of the census-registration law (minjeokbeop) in 1909, all subjects came to register a surname and clan seat in the household register. In the process of newly choosing a surname, it is said, those who had none often picked a widely known great surname or a famous clan seat.
Why are Kim, Lee, and Park so common?
That Korean surnames today cluster so heavily in a few great surnames rests on this history. In the social climbing of the late Joseon period and the spread of surnames in the late 19th and early 20th centuries, a concentration toward famous surnames and clan seats took place. So sharing a clan seat does not necessarily mean descent from a single bloodline. It is a point at which the story of "our family's roots" can be read anew — as a trace of how greatly Joseon society was overturned.