The Jewish calendar's reference point is traditionally held to be about one year before the Creation of the world.
The Jewish calendar uses a calendar era anno mundi ("in the year of the world"), abbreviated AM. Interestingly, the beginning of "year 1" is not Creation, but about one year before Creation. This caused the new moon of its first month (Tishrei) to be called molad tohu (the mean new moon of chaos or nothing).
The Jewish calendar's epoch (reference date), 1 Tishrei 1 AM, is equivalent to Monday, October 7 3761 BCE in the proleptic Julian calendar, the equivalent tabular date (same daylight period) and is about one year before the traditional Jewish date of Creation on 25 Elul AM 1, based upon the Seder Olam Rabbah of Rabbi Yossi ben Halafta, a second century CE sage. Thus, adding 3760 before Rosh Hashana or 3761 after to a Julian or Gregorian year number after 1 CE will yield the Hebrew year. For earlier years there may be a discrepancy (see: "Missing Years" in the Hebrew Calendar).
The Jewish year starting on Rosh Hashanah, 1 Tishrei, 5769 AM is equivalent to 29 September 2008.
The year number on the Jewish calendar represents the number of years since creation, calculated by adding up the ages of people in the Bible back to the time of creation. However, this does not necessarily mean that the universe has existed for only 5700 years as we understand years. Many Orthodox Jews will readily acknowledge that the first six "days" of creation are not necessarily 24-hour days (indeed, a 24-hour day would be meaningless until the creation of the sun on the fourth "day"). For a fascinating (albeit somewhat defensive) article by a nuclear physicist showing how Einstein's Theory of Relativity sheds light on the correspondence between the Torah's age of the universe and the age ascertained by science, see The Age of the Universe.