Orthodox Judaism is not a unified movement with a single governing body, but many different movements adhering to common principles. All of the Orthodox movements are very similar in their observance and beliefs, differing only in the details that are emphasized. They also differ in their attitudes toward modern culture and the state of Israel. They all share one key feature: a dedication to Torah, both Written and Oral.
Origins of the Movement
Historically, there was no such thing as Orthodoxy; in fact, you find the particular term is used primarily in North America (elsewhere the distinction is primarily between “more observant” and “less observant”). The specific term “Orthodox Judaism” is of rather recent origin and is used more as a generic term to differentiate the movements following traditional practices from the Liberal Jewish movements.
Orthodox Judaism views itself as the continuation of the beliefs and practices of normative Judaism, as accepted by the Jewish nation at Mt. Sinai and codified in successive generations in an ongoing process that continues to this day.
Orthodox Judaism believes that both the Written and Oral Torah are of divine origin, and represent the word of G-d. This is similar to the view of the Conservative movement, but the Orthodox movement holds that such information (except for scribal errors) is the exact word of God and does not represent any human creativity or influence. For the Orthodox, the term “Torah” refers to the “Written Law” as interpreted by the “Oral Law,” interpreted in turn by the Rishonim (Medieval commentators), and eventually codified in the Codices: R. Joseph Karo's Shulhan Arukh and/or R. Moshe Isserlis's Mapah (printed as parenthetical text in the Shulhan Arukh). As practical questions arise, Orthodox authorities apply the Halachic process (the system of legal reasoning and interpretation described in the Oral Torah) using the Torah (both Oral and Written) to determine how best to live in accordance with G-d's will. In this way, Orthodoxy evolves to meet the demands of the times. An excellent summary of the core beliefs of Orthodox Judaism may be found in the Rambam's 13 Principles of Faith.
One of the hallmarks of Orthodox Jews is an openness (and encouragement) to question what it is that G-d requires of us, and then to answer those questions within the system that G-d gave us.
In addition, among the major movements only Orthodoxy has preserved the “mystical” foundations of Jewish theology, most obviously in the Chasidic movements though no less so in many yeshiva movements, both Ashkenazi and Sephardi.