Descendents of Aaron the priest enjoy special privileges.
Reprinted from The Jewish Religion: A Companion, published by Oxford University Press.
A Kohen (plural, Kohanim) is a 'priest,' a descendant of Aaron the priest. The priestly case officiated in the Temple and have certain functions to perform even after the destruction of the Temple. The Jewish family name, Cohen, usually denotes that its members were priests.
In Temple times no one was admitted to the priesthood unless he could prove his priestly descent. In later times rigorous proof was no longer possible, so that Kohanim today act as such on the basis of presumptive status—the mere fact that a family tradition believes that it is formed of Kohanim is sufficient to establish its status as such.
A Kohen may not come into contact with a corpse unless it is of a near relative (Leviticus 21:1-4); and he may not marry a divorcee (Leviticus 21:7). It is the Kohen's privilege to be the first of the persons called to the reading of the Torah. Where a Kohen is present at the table he has the right to recite the grace after Meals, though he can waive this right if he chooses. Kohanim also recite the priestly blessing in the synagogue.
These rules are followed by all Orthodox Jews. Reform Jews reject the laws concerning Kohanim in the rather fanciful belief that they tend to perpetuate a caste system in Judaism. Conservative Jews are less categorical in the matter but many Conservative rabbis also hold that the laws about the Kohanim are in abeyance today, especially since, nowadays, those who claim to be Kohanim only enjoy their privilege by presumptive status.