Answer:
   
   The Tosafot were composed by many scholars in different schools
   throughout the 12th and 13th centuries. They probably originated as
   students' notes of the discussions that took place in the Talmudic
   academy [=Yeshivah]. As students moved from one yeshivah to another
   they would assemble personal lists of the Tosafot of their various
   teachers. Some of the most prominent contributors to the Tosafot were:
   
   Rabbi Jacob ben Meir (Rabbenu Tam). 1100 - 1171.
          Rashi's grandson, lived in the French town of Ramerupt.
          
   Rabbi Samuel ben Meir (The Rashbam). 1080 - 1158.
          A grandson of Rashi's and the brother of Rabbenu Tam. In
          addition to his contributions to the Tosafot, he composed a
          famous commentary to the Torah that is distinguished by its
          scholarly objectivity in restricting itself to the plain,
          contextual meaning of the text without imposing the traditional
          Rabbinic interpretations.
          
   Rabbi Isaac of Dampierre (The Ri). 
          A nephew of Rabbenu Tam and the Rashbam, he lived in France
          during the 12th century; One of the most prolific of the
          Tosafists.
          
   Rabbi Samson [ben Abraham] of Sens.
          He lived in France during the latter 12th and early 13th
          centuries, and eventually moved to Jerusalem. He was the most
          important disciple of Rabbi Isaac of Dampierre. In addition to
          his Tosafot he composed a commentary to the two orders of the
          Mishnah for which there is no Babylonian Talmud.
          
   Rabbi Meir [ben Barukh] of Rothenburg. 1225 - 1293.
          Rabbi Meir made important contributions to Jewish civil law,
          and his many students diligently collected his customs,
          responsa and rulings, often comparing them with the material in
          the important Spanish codes of Jewish law.
          
   Unlike the explanatory commentaries, such as Rashi's, the Tosafot do
   not attempt to provide a full elucidation of the Talmud text. Rather
   they focus on particular issues in the Talmud or in Rashi's commentary
   which they explore in depth. They often propose alternative readings
   or interpretations to the ones presented by Rashi.