To further illuminate, ever so slightly, Octavius' splendid response... although many, many genealogical records were lost whenthe Romans under Titus destroyed the Temple in A.D. 70, many Jews continued and continue today to maintain exacting historically accurate records. The 'kohanim' referenced in Octavius' post actually points directly to a family name today... the Cohens.
As an aside, a small but dedicated group are working today, within Israel, to re-establish the Temple on Temple Mount. They have re-created all of the utensils and clothing necessary to establish the Priesthood. They have even sent emmissaries to some cattle companies, near my home here in the western U.S. searching for the perfect "Red Heifer" which will be absolutely required when the Temple is restored. At any rate, several Cohen's have had their ancestry verified and have been trained or are being trained as Chief Priest and Priests as well as the other Levites that will be required. this is preparation for the coming of the Jewish Meshiach or Messiah...