The Rosh is one of the most crucial Rishonim on the Talmud. He is printed in the back of almost every Talmud, and is useful both as a halachacist, and also as one of the last tosafists he often clarifies and explains their position.

He is the bridge between Talmudic commentary and Halachic works. His son, the Tur (Yaakov ben Asher) is the author of the Arba Turim, which is the predecessor of the Shulchan Aruch. The Tur also wrote ‘piskei harosh’ which is a summary of the halacha derived from the Rosh’s Talmudic commentary and is also printed in the back of most editions of the Talmud.

The Rosh was also the bridge between the Ashkenazi and Sefardi worlds, forced as he was to resettle and move from Northern Europe to Spain. His commentary is therefore an interesting commentary on the different customs of East and West at the time. R’ Yosef Karo uses him as the ‘Ashkenazi’ amonst his three main sources (the other two being the Rif and Rambam).

This is from the OU:

Rabbi Asher ben Yechiel – The “Rosh”
(1250-1328)

The commentary on the Talmud authored by the Rosh, Rabbi Asher ben Yechiel, is found in the back of nearly every edition of the Babylonian Talmud. The popularity of his commentary reflect his clarity of thought and expression, and his halachic authority as the close disciple of the Maharam, Rabbi Meir of Rothenberg (1215-1293). The Maharam was the last of the German Baalei Tosafos. Thus, his student, Rabbi Asher ben Yechiel, assimilated and represented the Ashkenazic tradition of Torah learning.

Because of the savage activities of the Crusaders, Rabbi Asher fled to Spain in 1306. His first stop was Barcelona, where the Rashba gave him a cordial welcome. He then moved on to Toledo, where he became Chief “Dayan,” Judge in matters of Jewish Law, and Chief Rabbi of the community.

The Rosh was not pleased by the Spanish approach to Torah learning, which he saw as attributing excessive value to philosophy and other secular studies. He felt that the unavoidable consequence of the Spanish approach would be the “watering down” of the focus on the Talmud, the true source and repository of Jewish wisdom. Yet, by the sheer greatness of his Torah ideas, the influence of Ashkenaz began to penetrate the Spanish Torah world.

The halachic opinions of the Rosh were binding upon Ashkenazic Jewry. But Spanish Jewry also began to take into account his opinions in their formulation of Jewish Law. Rabbi Yoseph Karo (1488-1575) in the “Bais Yoseph” and in the “Shulchan Aruch,” classics of Jewish Law, gave equal weight to the halachic opinions of the Rosh as to the opinions of the Sephardic Torah giants the Rif and the Rambam. His method in those cases where the opinions of Ashkenaz and Sepharad differed, was to decide the Law by “taking a vote,” as it were, among those three opinions, and the Law would be in accordance with two out of the three opinions.

Here is his biography from wikipedia

Asher ben Jehiel (Hebrew, אשר בן יחיאל, or Asher ben Yechiel, sometimes Asheri) (1250 or 1259 – 1328) was an eminent rabbi and Talmudist best known for his abstract of Talmudic law. He is often referred to as Rabbenu Asher, “our Rabbi Asher” or by the Hebrew acronym for this title, the ROSH (Hebrew, רא”ש, literally “Head”).

Biography

The Rosh was born in western Germany and died in Toledo, Spain. His family was prominent for learning and piety, his father Jehiel was a Talmudist, and one of his ancestors was Rabbi Eliezer ben Nathan (the RaABaN). Asher had eight sons, the most prominent of whom were Judah and Jacob, author of the Arba’ah Turim, a code of Jewish law. His primary teacher was the Tosafist Rabbi Meir of Rothenburg, then in Worms. In addition to his studies, ROSH worked in money lending, and according to his own statement, was independently wealthy.

In 1286, Emperor Rudolf I had instituted a new persecution of the Jews, and Rabbi Meir left Germany, but was captured and imprisoned. The ROSH raised a ransom for his release, but Rabbi Meir refused it, for fear of encouraging the imprisonment of other rabbis. Thereafter Rosh assumed Rabbi Meir’s position in Worms. He was, however, forced to emigrate (in all likelihood, a victim of blackmail by the government, aimed at acquiring his fortune). After leaving Germany, he first settled in southern France, and then in Toledo, Spain, where he became rabbi on the recommendation of Rabbi Solomon ben Aderet (RaShBA). Rabbenu Asher’s son Judah testified to the fact that he died in poverty.

Rabbenu Asher possessed “methodical and systematic” Talmudic knowledge, and was distinguished for his ability to adumbrate long Talmudic discussions. The ROSH, influenced by his teacher Rabbi Meir, was averse to lenient decisions in halakha, even when theoretically justified. (Several of his rulings which may appear lenient, are actually strictures: his decision against praying more than three times a day is, in fact, limiting. Similarly, his assertion that the phrase halacha le-Moshe me-Sinai – “an oral law revealed to Moses on Sinai” – does not always bear a literal meaning but often signifies a universally adopted custom, is not usually taken as a liberal interpretation.) The ROSH was, however, known for his independent legal reasoning: “We must not be guided in our decisions by the admiration of great men, and in the event of a law not being clearly stated in the Talmud, we are not bound to accept it, even if it be based on the works of the Geonim.” (For instance, the ROSH ruled that the liturgy of the Geonim, was not in violation of the Talmudic rule against change in the prayers.)

Rabbenu Asher was opposed to the study of secular knowledge, especially philosophy. He held that philosophy is based on critical research, whereas religion is based on tradition and the two are thus “incapable of harmonization”. He said that “none that go under her may return” – in fact, he thanked God for having saved him from its influence, and boasted of possessing no knowledge outside the Torah. He led the anti-Maimonists in Spain; he also attempted to issue a decree against the study of non-Jewish learning. One effect of this attitude was to limit his influence on secular Spanish Jewry. At the same time, within rabbinic circles, “he transplanted the strict and narrow Talmudic spirit from Germany to Spain”, and this, in some measure, turned Spanish Jews from secular research to the study of the Talmud.

Works

Rabbenu Asher’s best known work is his abstract of Talmudic law. This work specifies the final, practical halakha, leaving out the intermediate discussion and concisely stating the final decision. It omits areas of law limited to Eretz Yisrael (such as agricultural and sacrificial laws) as well as the aggadic portions of the Talmud. Asher’s son Jacob compiled a list of the decisions found in the work, under the title Piskei Ha-ROSH (decisions of the ROSH). Commentaries on his Halachot were written by a number of later Talmudists. In yeshivot, this work is studied as a regular part of the daily talmud study.

This work resembles the Hilchot of the Rif (Rabbi Isaac Alfasi) – also an adumbration – but differs in quoting later authorities: Maimonides, the Tosafists and Alfasi himself. Rabbenu Asher’s work superseded Alfasi’s within a short time and has been printed with almost every edition of the Talmud since its publication. This work was so important in Jewish law that Yosef Karo included the ROSH together with Maimonides and Isaac Alfasi as one of the three major poskim (decisors) considered in determining the final ruling in his Shulkhan Arukh.

Rabbi Asher also wrote:

* Orchot Chaim, an essay on ethics, written for his sons. It begins with the comment, “Distance yourself from haughtiness, with the essence of distancing.” Orchot Chaim is today an important text in mussar study.
* A commentary on Zeraim (the first order of the Mishnah) – with the exception of Tractate Berachot.
* A commentary on Tohorot (the sixth order of the Mishnah).
* Tosafot-like glosses on several Talmudic topics.
* A volume of responsa; see History of Responsa: Fourteenth century.

May His Soul be Bound in the Bonds of Eternal Life