I am not absolutely certain of anything but the Orthodox and original teachings (and I welcome corrections from anyone who can furnish proof), but here’s what I have gleaned about the major Jewish denominations, from information I gleaned from their own websites. Bear in mind that there are probably other groups as well, and some of them comprise subsets within them. (And I just might write an article about the subsets in Orthodox Judaism, some time in the future.) But basically, here is what the main groups are about.
A precautionary word: Orthodox Judaism believes that all Jews are Jews, no matter what they believe or do. We do not accept non-Orthodox teachings as valid, but a Reform (or Conservative, or Reconstructionist, or whatever) Jew is still a Jew.
Orthodox (the original Judaism): We affirm that the Torah — which includes both the Oral and the Written — was created by G-d and taught to Jews. Changes are never necessary, as Torah Law can be applied to any situation.
This has been proven to be true. I live that life, and I know it is true.
Conservative: They assert that the Torah was created by people (Jews) with Divine Guidance. They believe that the Written and the Oral Torah were created at different times, by men, despite the fact that it is impossible to understand either of them without the other.
Each generation, according to the Conservative, is likewise given Divine Guidance to make «necessary» changes.
Changes are instituted to bring their thinking in line with the philosophical thought of the dominant culture, despite the fact that Judaism has never done this. (Though, of course, numerous deviant cults among the Jews have always done this throughout history. None of those cults exist anymore as such, if at all. Some examples were: The Sadducees, the Boethusians, the Chitzonim, the Hellenizers, and the Christians. The last one on that list exists today only because that group split off entirely and became a completely different religion. At no time was any deviant philosophy accepted into mainstream Judaism.)
Conservative Judaism came about as a break-off group from Reform, by people who felt that Reform was leaving traditional Judaism at too fast a pace. They agreed with the Reform that Jews should leave traditional and original Judaism, but they felt it should be done more slowly. I’m not sure why.
Reform: Essentially, they believe that you get to decide what to believe. The Torah, they claim, is man-made entirely, and has been continually changed and adapted, despite all evidence to the contrary. Furthermore, despite the fact that the Torah and history show that Judaism has never tolerated dissenters to the Torah’s opinion, they have the chutzpah (gall) to claim that Judaism has always been pluralistic, that the Torah supposedly has never demanded «uniformity of belief or practice.» This, of course, is obviously baloney. Take one cursory look at almost any chapter of the entire Tanach (Jewish Bible), and you’ll find recriminations against people who have even slightly deviated from the Torah’s teachings, even if they adhered to everything else.
According to their latest platform, the Reform do not believe in Heaven or Hell, but they believe in the immortal soul. (I guess the immortal soul suffers from boredom for all eternity, since there is no actual place for the soul to go. <shrug>)
According to surveys taken in the past, a high percentage of Reform «rabbis» do not believe that G-d exists. Nevertheless, Reform believes, officially, that all Jews have an obligation (who created this obligation, I cannot imagine) to study «the traditions,» and then choose what works for them. Ultimately, however, the Reform say it is more a question of «keeping in touch with your roots,» than it is a way of serving Hashem. How this can be said to be the same as the original Judaism, I cannot imagine. It’s not even similar to Judaism in any way. (But yes, they are still Jews, if their mothers are Jewish.)
About ten years ago, a friend of mine who was Reform said it is against Reform teachings to disbelieve in G-d, and that it is impossible to be Reform and not believe in G-d. Maybe that was the official stance, but that was not the way it was in practice, since all my other Reform friends at the time did not believe in G-d. (And yes, they were and are still my friends.) Furthermore, a board of Reform rabbis has recently ruled that congregations who have removed G-d from their prayer books are still eligible for inclusion in the Reform Movement. Which is not to say that they condone or encourage it. They don’t. At least not officially. But as far as the official Reform platform goes, if you don’t believe in G-d, you are still religiously compatible with Reform beliefs.
Reconstructionist: They believe that if there is something you do not understand in the Torah, it means that you are correct, and the Torah is wrong. They say that in ancient times the Jews were very primitive, but we evolved, and kept changing our Torah, as the world’s societies evolved. (Again, it assumes that we keep in concert with Gentile philosophy, and that we have never known anything about morals and ethics that the Gentiles didn’t teach us. The truth is actually closer to the other way around, though not entirely.) Reconstructionists love to say «the past has a vote, not a veto.» If you don’t like what the Torah says, do what YOU like doing. Poof! Gone is the discipline the Torah teaches, because you don’t have to obey anyone at all except yourself, and, of course, the Gentile philosophers who have shaped modern-day thinking.
By contrast, Orthodox Judaism teaches that if you want to do the right thing, study the Torah. Everything you need to know about how to live properly is in the Torah. By Torah, we also include everything that the Rabbis have taught in concert with the Torah, because that is part of the original Torah. The Rabbis did not make anything up. All the Torah was created by G-d 2000 years before He created the universe. G-d gave us, the Children of Israel, all of the Torah, and we have transmitted the Torah down through the ages to this generation, and we shall, with G-d’s help, continue to do so with our own children, and theirs, and so on, as long as the universe shall last.
(Again, let me reiterate: All Jews are Jews, no matter what they believe or do, even if they belong to Reform, Conservative, or Reconstructionist congregations. The only exception is that if a Jew becomes an idolater he loses his connection to the greater Jewish «Family,» though he is still required to repent and return to Judaism.
For example, a Jew that becomes a Christian loses his identity as a member of the Jewish Family, even though he is still Jewish, and sins every moment that he is Christian. By Jewish Law, Christianity is permitted to Gentiles, but is considered idolatry for Jews. (Please note that the Jewish legal term of idolatry is better translated as «foreign adherence,» and applies to any religion or belief system that Judaism considers such, and therefore even applies to atheism, which technically has no «worship» at all.)
When he returns to Judaism, he returns to the «family.» A Jew who has joined or was born into a Reform, Conservative, or Reconstructionist group is still a Jew in all things.I repeat, he is still a Jew. However, the teachings of those groups do not constitute true Judaism. They are a deviation from the original Judaism. But their members are still Jews. I don’t care what you have heard or read that Orthodox Rabbis supposedly said. They never said it, and it isn’t true even if they had said it, which they didn’t. Jews are always Jews. See my article «Are Non-Orthodox Jews Still Jews?«)