Da'as Torah (or Da'at Torah, Da'as Toyreh) (Hebrew: דעת תורה, literally "Knowledge of Torah"), is a concept in Haredi Judaism according to which Jews should seek the input of rabbinic scholars not just on matters of Jewish law but on all important life matters, on the grounds that knowledge of the Torah aids everything in life.
In Contemporary Hareidi Judaism
Da'at Torah is not some Jewish equivalent to the Catholic doctrine of papal infallibility. Not only can rabbis make mistakes of judgment, there is an entire tractate of the Talmud, Horiut, predicated on the assumption that they can, that even the Sanhedrin is capable of erring, even in halachic matters. What Da'at Torah means, simply put, is that
those most imbued with Torah-knowledge and who have internalized a large degree of the perfection of values and refinement of character that the Torah idealizes are thereby rendered particularly, indeed extraordinarily, qualified to offer an authentic Jewish perspective on matters of import to Jews - just as expert doctors are those most qualified (though still fallible, to be sure) to offer medical advice.
That which Poskim cannot prove from an explicit source, is decided upon by a thought-process which has been tuned and refined by tens of thousands of hours of Torah study (and with elderly Talmidei Chachamim even hundreds of thousands of hours) which enables them to perceive where the pure truth lies. This process is called Da’as Torah - an opinion born out of Torah thought. Their thought-process has not been affected by secular and non-Jewish ways of thinking ... 
Accordingly, Hareidi Judaism espouses a belief that Jews, both individually and collectively, should seek out the views of the prominent religious scholars of the generation (called Gedolim, "great ones", or gedolei Torah or gedolei Yisrael) not just on matters of Jewish law or matters "of religion", but in every aspect of life, from issues of health to secular politics.
Further whereas in declaring matters of Jewish law rabbinic authorities are required to render decisions based on precedents, sources, and Talmudic principles of analysis, a rabbinic authority has greater latitude when declaring Da'as Torah than when defining a halachic opinion. While a halachic opinion requires legal justification from recognized sources, simple Da'as Torah is regarded as being of a more subtle nature and requires no clear legal justification or explicit grounding in earlier sources. Indeed, different authorities may offer diametrically opposed opinions based on their own understanding (Feitman 1995).
In Hasidic circles, a rebbe is often regarded as having extraordinary spiritual powers and is sought for personal advice in all pursuits of life by his followers. The views and edicts of a rebbe are considered to be an expression of Da'as Torah.
Theories about its background
Some observers, such as Dr. Jacob Katz, feel that the idea that these are de facto binding on the whole community is a novelty within the history of Judaism.
Prior to the modern period, rabbis functioned as the primary leaders of every Jewish community. Lay leaders served under the general guidance of the rabbinical leadership regarding religious issues, and often on political matters as well. This was not understood to mean that rabbis, even gedolim, were infallible, but simply that they were seen as the best qualified leaders for the Jewish people (Feitman 1995).
Some argue (e.g. Kaplan 1980) that with the rise of modernity and the wider availability of secular knowledge (and a reduction of commitment to religion), various groups, including groups within the "Orthodox" world, raised challenges to the exclusive leadership role of the rabbis. These theorists suggest that to some degree, this generated a backlash in the Haredi world, intensifying the Da'as Torah concept to imply near infallibility for gedolim.
Avi Shafran disagrees that the concept is a new one. He writes:
The phrase Da'at Torah may be a relatively new one, but the insinuation that the concept it reflects is some sort of modern invention by "unmodern" Jews is absurd. "Emunat chachamim," or "trust in the judgment of the Torah-wise," has been part and parcel of Jewish tradition for millennia. The Talmud and Jewish history are replete with examples of how the Jewish community looked to their religious leaders for guidance about social, political and personal decisions - decisions that, as believing Jews, they understood must be based on authentic Torah values.
R. Nahum Rabinovich, in a recently translated article in Hakirah, argued that the practice found in some Haredi circles of blindly following the advice of rabbinic leaders for all personal decisions is not related to the mishnaic concept of emunat chachamim.
The Da'as Torah concept is the principle that guides the Haredi political parties in Israel such as the Ashkenazi Agudat Israel (working under the tutelage of the present Gerrer Rebbe, Rabbi Yaakov Aryeh Alter) and Degel HaTorah (presently guided by Rabbi Yosef Shalom Eliashiv, both individually and during their alliance together as United Torah Judaism, and the Sephardi Shas party (guided mainly by Rabbi Ovadia Yosef.) National Religious Party of Israel also has a centralized board of Rabbis, such as Rabbanim Shlomo Aviner, Dov Lior and Mordechai Eliyahu that formulate its policies, although its politicians have greater flexibility in devising such policies.
- Feitman, Rabbi Yaakov. "Daas Torah: Tapping the Source of Eternal Wisdom". In: Torah Lives, ed. Nisson Wolpin. Brooklyn, NY: Mesorah Publications, 1995. Pg ix-xxviii. ISBN 0-89906-319-5.
- Kaplan, Lawrence, "Daas Torah: A Modern Conception of Rabbinic Authority," in Rabbinic Authority and Personal Autonomy, ed. Moshe Z. Sokol (Northvale: Jason Aronson, 1992), 1-60. 
- Kaplan, Lawrence, "Daat Torah: A Modern View of Rabbinic Authority," in Zev Safrai and Avi Sagi, eds., Between Authority and Autonomy in Jewish Tradition, 105-145. Tel Aviv: Hakibbutz Hameuhad, 1997 [Hebrew].
- Kaplan, Lawrence, "Rabbi Isaac Hutner's 'Daat Torah Perspective' on the Holocaust: A Critical Perspective," Tradition 18, no. 3 (Fall 1980): 235-248. 
- Katz, Jacob, "Da'at Torah- The Unqualified Authority Claimed for Halachists," The Harvard Law School Program in Jewish Studies The Gruss Lectures - Jewish Law and Modernity: Five Interpretations, October 26 - November 30, 1994, Copyright 1997 The President and Fellows of Harvard College. 
- Pelta, Baruch. "Interview with Professor Lawrence Kaplan," The Seforim Blog, 21 October 2009, .