I was recently asked what differentiates Orthodox, Reform and Conservative Judaism.
Without going into philosophical or theological specifics, my answer was: Choice.
Orthodox Judaism (including Hasidut) teaches that an oral tradition of interpretation accompanies the “written” Torah. Historically, that became the Talmud. But it also implies an ongoing process of interpretation. New questions come up in every Jewish setting, in every generation. For example, can we turn on electric lights on Shabbat? We can’t find any direct reference to “electricity” in either Torah or Talmud. In the absence of the Sanhedrin — a “congress” of rabbis arriving at answers by deliberation — addressing the question required a reliable authority. Not every “authority” — Maimonides or Rav Soloveitchik, for example — is universally accepted. So, even within Orthodoxy, there can be a range of accepted opinions. But the underlying attempt is to align individual choices with the Divine Will expressed in Torah and elucidated further in Talmud.
Even where there’s disagreement, multiple individuals can be considered “authorities.” Both Hillel and Shammai, for example, were looked on as “authorities,” even though they disagreed over many issues.
When Jesus allowed his followers to pick ears of corn on Shabbat , the rabbis (“Perushim” in the text) protested. Jesus, not accepting the authority of the rabbis to mandate how Shabbat could be observed, responded with a rationale and a declaration that he had the authority to mandate how the mitzvah of “kal melechet avodah lo ta’asu” — “Don’t do work on Shabbat” — can be implemented.
Without taking either side, I’m pointing out that this was the fundamental, essential break with “rabbinic” Judaism. Jews are often faced with the question: “Why did the Jews reject Jesus?” The answer is: The rejection was mutual.
In Orthodox Judaism, then, one doesn’t make individual choices. When a question arises, a Jew is supposed to seek a rabbinic authority for a solution or resolution. The Shulchan Aruch was written to provide easier reference to acceptable practices.
Reform Judaism is all about personal, individual choice:
“…[Reform Judaism] strongly recommends individual study of the traditional practices; however, the adherent is free to follow only those practices that increase the sanctity of their relationship to God.” 
Different generations in Reform emphasize different themes. A popular current theme is “social action.” A few years ago, “spirituality” was a topic that Reform avoided. For example, when faced with the question of including “Jewish Science” as part of the Reform platform in the 1920’s, the solution was to “table the discussion.” It was never resolved. In recent years, “spirituality” has become far more acceptable as a theme for discussion and, at times, practice.
The preference for personal choice might seem like a radical break with Jewish tradition. In many ways, it is. But Reform teachers always refer back to traditional thinkers, writers and practices. So, the attempt to remain within Jewish tradition — albeit, within whatever parts we might choose — is clear.
Conservative Judaism, like Reform, believes that the practice of Torah can be adjusted to ever-changing environments in every generation. The difference is that Conservative practice is defined by the Rabbinical Assembly:
“The RA was founded in 1901 to shape the ideology, programs, and practices of the Conservative movement. It…oversees the work of the Committee on Jewish Law and Standards for the Conservative movement.” 
Conservative Judaism allows for changes to be made in practice, but only by a recognized body of rabbis. Rabbi Isaac Klein’s “A Guide to Jewish Religious Practice” functions as a kind of “Conservative Shulchan Aruch.”
I can’t write here about Reconstructionist or Renewal Judaism. I don’t feel familiar enough with them in detail. To the extent that they allow for individual choice, they are philosophically Reform (or parallel to it). To whatever extent they might mandate specific ideas or practices as guidelines that define Jewish identity, they might be considered more like Conservative Judaism.
As an adult returning to Judaism, I was initially most comfortable in “Modern Orthodox” services; “Young Israel,” for example. These were the kind practiced at the synagogue in which I grew up. It was not “Young Israel,” but it was “Modern Orthodox.” After that, I thought of myself as a “Conservative Jew” for many years because I felt that Conservative Judaism gave me greater intellectual leeway, simply through the lack of strict definition. Now, I realize that to the extent that I take upon myself the right to make personal choices, I’m philosophically a “Reform Jew,” but an eclectic one who includes Conservative practices and Hasidic study.
In my own mind, both Christianity (in all its permutations) and Islam are “Reform Judaisms,” too. There is an essential unity in what all teach and believe, but we differ in the sources of authority we recognize. Ultimately, I think we all look to the same God for our ultimate validation.
 Mark 2:23-28