Rashi wrote the first comprehensive commentary of almost the entire Tanakh (Hebrew Bible, Old Testament). There are a small number of commentaries that bear his name that were not authored by him, but by his students.
Rashi also wrote the first comprehensive commentary of almost the entire Talmud. His work became such a standard that it is included in all printed versions of the Talmud. His commentary is always situated towards the middle of the opened book display; i.e. on the side of the page closest to the binding. The semi-cursive font in which the commentaries are printed is often referred to as "Rashi script." This does not mean that Rashi himself used such a script, only that the printers standardly employ it for commentaries. And Rashi's were the commentaries par excellence to both the Bible and the Talmud. Rashi's Commentary which covers almost the whole of the Babylonian Talmud, has been printed in every version of the Talmud since the first Italian printings.
His commentary attempts to provides a full explanation of the words, and of the logical structure of each Talmudic passage. Unlike some other commentaries, Rashi does not paraphrase or exclude any part of the text, but carefully elucidates the whole of the text. Rashi also exerted a decisive influence on establishing the correct text of the Talmud. He compared different manuscripts and determined which readings should be preferred.
We do not possess Rashi's commentary for every tractate of the Babylonian Talmud, and a few of the printed commentaries attributed to him were composed by others. In some instances, the text indicates that Rashi died before completing the tractate, and that it was completed by a student. This is true of the tractate Makkot, the concluding portions of which were composed by his son-in-law Rabbi Judah ben Nathan and of Bava Batra finished (in a much wordier and detailed style) by his grandson, Rabbi Samuel ben Meir (Rashbam), one of the prominent contributors to the Tosafot. It is probably a sign of the success of Rashi's achievement that no subsequent scholar, until Adin Steinsaltz in the late 20th century, tried to compose another comprehensive explanatory commentary.
Rashi's commentaries are of especial interest to secular scholars because he tended to translate unfamilar words into the spoken French of his day. As such, his commentaries offer an interesting insight into the vocabulary and pronunciation of medieval French.
See also Tosafists.