Rashi was the outstanding Biblical commentator of the Middle Ages. He was born in Troyes, France, and lived from 1040 to 1105, surviving the massacres of the First Crusade through Europe. He was a fantastic scholar and studied with the greatest student of Rabbenu Gershom of Mainz
At twenty-five, he founded his own academy in France. Rashi's commentary on the Bible was unique. His concern was for every word in the text which need elaboration or explanation. Moreover, he used the fewest words possible in his commentaries.
Most of his explanations were not written by him. Apparently, students would ask him questions about the text, or he would rhetorically ask questions about specific words, and a student would write his short, lucid answers in the margin of the parchment text. These answers comprise Rashi's commentary. We now have the answers, but the trick to studying Rashi is to figure out what the problem was with the text or the grammar of a given word.
Besides explaining individual words, Rashi also made use of the the great oceans of midrash. However, instead of just quoting the early rabbis, Rashi applied the stories specifically to the Bible text; often abridging them. He assumed that his students knew the midrash; he just emphasized its immediate relevance to the TaNaCH.
Rashi is also important for students of French. Many words in the Bible were unknown to Rashi's students, and obviously there would ask what a particular word meant and Rashi would give the answer in Old French using Hebrew transliteration. These transliterations provide important insights into the development of French and its pronunciation.
The original printed Bible text by Daniel Bomberg in 1517 included Rashi's commentary. That commentary became so popular that there are now more than 200 commentaries on his commentary. It is assumed in traditional circles that when you read the TaNaCh, you also read Rashi.
Rashi's commentary on the Talmud was even more important than his TaNaCh commentary. The Talmud was written in legalese: terse, unexplained language with no punctuation. Rashi provided a simple explanation of all Gemarra discussions. He explained all of the terse phrases; he explained the principles and concepts assumed by the sages who put together the Gemarra.
His simple, brief explanations for practically every phrase of the Gemarra made the Talmud understandable to the non-scholar. It became an instant best seller, and, to this day, it is unthinkable to study Talmud without studying Rashi's commentary at the same time.
Rashi's explanations and commentaries on the Talmud were so important that for almost a hundred years after his death, Talmud students in France and Germany concentrated their brilliant minds on discussing and elaborating on Rashi's commentary. Just as the monks were concentrating on deep philosophical discussions of Christian theology, France's Jewish scholars were focusing on the Talmud and its text. Their complicated (and sometimes convoluted) commentaries were called Tosafot (Additions). The scholars who created these additions were called the Tosafists (Those Who Added).
The most famous of these Tosafists was Rashi's grandson, Rabbenu Tam, who frequently disagreed with his grandfather. Today on every page of Talmud you can find Rashi's commentary surrounding the text on the inside of the page, and the Tosafot surrounding the text on the outside of the page.
Sources: Gates to Jewish Heritage