The Tosafot or Tosafos (lit. "additions") are medieval commentaries on the Talmud. They take the form of critical and explanatory glosses, printed, in almost all Talmud editions, on the outer margin and opposite Rashi's notes. The authors of the Tosafot are known as Tosafists ("ba'ale ha-tosafot").
Tosafoth were the sons-in-law, grandchildren, and students of Rashi (and their students), and "they" wrote a commentary on the Talmud that is often at odds with Rashi's earlier commentary. Sometimes ...