List of Tosafists
Tosafists were medieval rabbis from France and Germany who are among those known in Talmudical scholarship as Rishonim (there were Rishonim in Spain also) who created critical and explanatory glosses (questions, notes, interpretations, rulings and sources) on the Talmud. These were collectively called Tosafot ("additions"), because they were additions on the commentary of Rashi. The Tosafists lived from the 12th century to the middle of the 15th century and the Tosafot are a compilation of the questions, answers and opinions of those rabbis. The Tosafot are very important to the practical application of Jewish law because the law will differ depending on how the Talmud is understood and interpreted.
Each generation of the Tosafists would add to those compilations and therefore there are many different versions of the Tosafos. In addition each compilation of the Tosafos did not contain everything that was said by the Tosafists on the subject so compilations will differ in what they say. For this reason some things that were said by the Tosafists will be found only in obscure versions of the Tosafos.