It is very difficult to define what is essential to Judaism, but across the world and throughout time, some basic beliefs and practices have remained constant. The Rabbi Maimonides attempted to describe the thirteen core beliefs of Judaism (Thirteen Principles of Jewish Faith), and this is probably still the best resource, though some of the teachings of Reform Judaism may allow for more of an interpretive approach to the Principles.
The Thirteen Principles of Jewish Faith include the belief in the Creator, G-d (the "o" is omitted out of respect,) who is uniform, eternal, omniscient, and unaffected by physical happenings. Further, Judaism is the worship of this one G-d and no others. G-d speaks to man through prophecy and inspired the writings of the Talmud and Torah. The Torah is immutable, though the Reform movement allows for interpretation based on the context, holding that there is an essence of truth in passages which may appear "out of date." Moses is believed to be the highest prophet of Judaism. Furthermore, the Jewish faith teaches the resurrection of the dead and divine reward or retribution for actions during this life. Jewish people also believe in the coming of the Messianic Era, a time of total harmony with G-d and Their will.
These are some of the core teachings of Judaism, though as I have mentioned, there are many more particularities based on the particular sect of Judaism as well as a person's larger culture.