The whole western culture -- as Thomas Cahill points out in one of his great books "The Gifts of the Jews" -- owes a great amount of gratitude to this "Tribe of Desert Nomads." Before the Jewish patriarch, Abraham, the whole of the world's culture thought in of time in a circular sense ( what was will be again ). After Abraham came the idea was born that there was an individual free will; one can choose ones own destiny and not bound to a circular history. At this same time was introduced the idea that there was one god and one god only, the G-d. Up to this time, there was the belief that there were many gods, a whole pantheon, one for every object and thought that existed. Another addition to the western culture was the idea of a day of rest. G-d created the heavens and the earth in six days, and rested on the seventh. Although a day to human-kind is not always the same in reference to G-d (a day can often translates as a 1000 human years, sometimes more), G-d commanded that the seventh day of the week be remembered and kept holy. As a result, a day of rest was introduced, and, now, often in Western culture even two days are given.
The Sabbath, the seventh day of the week, the day of rest, is observed by the Jewish community on Saturday. In most Christian circles, the Sabbath is observed on Sunday. This was a result of a decision by the very early Roman Catholic Church, of which, no offence intended, was a faulty decision. Most christian scholars are fully aware of the error, but few -- unfortunately -- take the courage to point it out, or try to change it back.
Observance of the Sabbath may be one of the most important things in Judaism! On the Sabbath, one refrains from all worldly work. In fact, one does not cook, bathe, light a candle, etc., once the Sabbath begins (Friday evening at Sunset). It is a day of total leisure, family togetherness, and getting close to G-d. Before Sunset on Friday, it is customary for all washing, cleaning, cooking, etc., to be completed. Father, and the older children have gone to Synagogue, and Mother lights the candles -- with blessings -- with the younger children. When Father comes home from evening synagogue, everyone gathers for the evening meal. The Sabbath is often accompanied by a visit to synagogue and/or the study of Torah (the first five books of the Hebrew bible -- often called the First or Old Testament ). Sabbath refreshes and energizes the soul for the upcoming week. Especially in these days when a young adult can not easily devout a portion of each day to the study of Judaism, the Sabbath is increasingly important and vital to any Jew.
Christianity -- which sprouted from Judaism -- often only recognizes ten commandments from G-d. These are the ten words that Moshe (Moses) brought down to the newly freed children of Israel from Mount Sinai after their escape from Egypt. But, a careful study of the first five books of the Hebrew Bible reveals that truly, there are 613 commandments from G-d. Not all, now, apply, however. This is because there are a number which only apply when the Temple is present. (There is only one Temple, which resided in Jerusalem until its final (second) destruction at the beginning of the 1st millennium, C.E.)
Why are there no monks or nuns in Judaism? Simply, Judaism discourages people from separating themselves from the community. Judaism believes that one should participate in the community around them, even that they should help those who are ill or in need. On top of this, Judaism discourages the extreme zealot. Normality, not to be cliche, should be the norm.
There are various degrees of devotion to Judaism. Ranging from the Ultra-Orthodox Jew to the Reconstructionist Jew. Unlike the other major religions, there is very little separation between the various groups of Judaism.. The differences are mostly as far as how observant one is to the faith. Note, that a convert to Judaism under a Recostructionist Rabbi may not be recognized by an Orthodox or Ultra-Orthodox group... But a conversion by an Ultra-Orthodox rabbi is almost always recognized by all other groups. Hasidic Jews have a bit of a "mystic" leaning in their belief. "Mystic" is not exactly a proper word, here, but may be the closest word you can find in the English language. Hasidics find that one can find even more enlightenment in G-d with such simple things as the study of the numerical value of a word or phrase in the Torah. Also, meditation (a deep thought, or concentration on an idea) can be very revealing.
Another difference between Christian faiths and Judaism is the view of sex. In Judaism, never should one be ashamed of sex, as can be found in Christian faiths. Sex is a gift, a blessing of G-d. Sex, the bringing together of two opposite sexes into one is much like making one-self more like G-d, humbly speaking, of course. G-d is without sexual distinction, both having male and female characteristics. But we often refer to G-d in the male gender in our language because we often refer to G-d exhibiting those characteristics characteristic of the male sex. Sex is a union of the two human halves, and like G-d, at this moment, even mankind can help create life. But, like any gift from G-d, it should never be abused and taken lightly.
As stated before, "What is Judaism?" is not easy to answer. The text above can only scratch the surface. Judaism is truely a very rich religion. Not only that, but the Jewish people are truely rich, not materially (for they may have suffered more than any other peoples in this physical world) but spiritually. Not only this, but the non-Jew -- Gentiles -- living in Western Culture owe much of their very culture to this small tribe of desert nomads.