The disciples of Hillel the Elder, around the end of the Second Temple period, whose overall approach to the halakha was more lenient than that of the rival school, Beit Shammai. These disciples, though lenient and modest, won the hearts of the sages, and the office of Nasi was held by scions of the Hilllelite family for over 400 years, until the Sanhedrin ceased to exist. In disputes with Beit Shammai, Beit Hillel prevailed in most cases.
Read more about the rivalry between Beit Shammai and Beit Hillel in the Jewish Encyclopedia.