Orthodox Judaism believes that both the Written and Oral Torah are of divine origin, containing the exact words of G-d without any human creativity or influence.
Rambam's 13 Principles of Faith are an excellent summary of the core beliefs of Orthodox Judaism.
1. I believe with perfect faith that G-d is the Creator and Ruler of all things. He alone has made, does make, and will make all things.
2. I believe with perfect faith that G-d is One. There is no unity that is in any way like His. He alone is our G-d. He was, He is, and He will be.
3. I believe with perfect faith that G-d does not have a body. Physical concepts do not apply to Him. There is nothing whatsoever that resembles Him at all.
4. I believe with perfect faith that G-d is first and last.
5. I believe with perfect faith that it is only proper to pray to G-d. One may not pray to anyone or anything else.
6. I believe with perfect faith that all the words of the prophets are true.
7. I believe with perfect faith that the prophecy of Moses is absolutely true. He was the chief of all prophets, both before and after Him.
8. I believe with perfect faith that the entire Torah that we now have is that which was given to Moses.
9. I believe with perfect faith that this Torah will not be changed, and that there will never be another given by G-d.
10. I believe with perfect faith that G-d knows all of man's deeds and thoughts. It is thus written (Psalm 33:15), "He has molded every heart together, He understands what each one does."
11. I believe with perfect faith that G-d rewards those who keep His commandments, and punishes those who transgress Him.
12. I believe with perfect faith in the coming of the Messiah. How long it takes, I will await His coming every day.
13. I believe with perfect faith that the dead will be brought back to life when G-d wills it to happen.
In terms of practice, Orthodox Jews strictly follow the Written Torah and the Oral Law as interpreted by the Rishonim (Medieval commentators) and codified in the Codices (R. Joseph Karo's Shulhan Arukh and R. Moshe Isserlis's Mapah).
The term "Orthodox" Judaism only emerged as a result of the growth of new branches of Judaism. 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.
It follows that "Orthodox" is not a unified movement with a single governing body, but rather many different movements that all strictly observe Judaism. While all orthodox movements are similar in their beliefs and observance, they differ in the details that are emphasized and in their attitudes toward modern culture and the State of Israel. "Modern Orthodox" tend to be a bit more liberal and more Zionistic. "Ultra-Orthodox", including Yeshivah movements and the Chasidic sect, tend to be the least open to change and the most critical of modern society.
Chasidism, founded in Europe by the Ba'al Shem Tov, believes that acts of kindness and prayer could be used to reach God, as opposed to the older view that one could only become a righteous Jew through rigorous learning. The word Chasid describes a person who does chesed (good deeds for others). Chasidic Jews dress distinctively, live separately from modern society, and are dedicated to strict observance of Jewish Law.
Orthodox Judaism is the only movement that has preserved the "mystical" foundations of Jewish theology.
