The House of Hillel (Beit Hillel) and House of Shammai (Beit Shammai) were, among Jewish scholars, two schools of thought during the period of tannaim, named after the sages Hillel and Shammai (of the last century BCE and the early 1st century CE) who founded them. These two schools had vigorous debates on matters of ritual practice, ethics, and theology which were critical for the shaping of the Oral Law and Judaism as it is today.
The Mishnah mentions the disagreement of Hillel and Shammai as one which had lasting positive value:
A disagreement which is for the sake of Heaven will be preserved, and one which is not for the sake of Heaven will not be preserved. What is a disagreement that is for the sake of Heaven? The disagreement of Hillel and Shammai. What is not for the sake of Heaven? The disagreement of Korah and his congregation.[1]
In most cases, though not always, Beit Hillel's opinion is the more lenient and tolerant of the two. In nearly all cases, Beit Hillel's opinion has been accepted as normative by halacha, and is the opinion followed by modern Jews.