Hillel and Shammai
In the first century BCE, Babylonian born Hillel (later known as Hillel the Elder) migrated to the Land of Israel to study and worked as a woodcutter, eventually becoming the most influential force in Jewish life. Hillel is said to have lived in such great poverty that he was sometimes unable to pay the admission fee to study Torah, and because of him that fee was abolished. He was known for his kindness, gentleness, concern for humanity. One of his most famous sayings, recorded in Pirkei Avot (Ethics of the Fathers, a tractate of the Mishnah), is "If I am not for myself, then who will be for me? And if I am only for myself, then what am I? And if not now, when?" The Hillel organization, a network of Jewish college student organizations, is named for him. Hillel and his descendants established academies of learning and were the leaders of the Jewish community in the Land of Israel for several centuries. The Hillel dynasty ended with the death of Hillel II in 365 CE.
Hillel the Elder’s friendly adversary was Shammai, a native of the Land of Israel about whom little is known except that he was a builder, known for the strictness of his views. He was reputed to be dour, quick-tempered and impatient. Both lived during the reign of King Herod (37-4 BCE), an oppressive period in Jewish history because of the Roman occupation of the Land of Israel. Shammai was concerned that if Jews had too much contact with the Romans, the Jewish community would be weakened, and this attitude was reflected in his strict interpretation of Jewish law. Hillel did not share Shammai's fear and therefore was more liberal in his view of law.
Hillel was the more popular of the two scholars, and he was chosen by the Sanhedrin, the supreme Jewish court, to serve as its president. While Hillel and Shammai themselves did not differ on a great many basic issues of Jewish law, their disciples were often in conflict. The Talmud records over 300 differences of opinion between Beit Hillel (the House of Hillel) and Beit Shammai (the House of Shammai). The Rabbis of the Talmud generally sided with the rulings of the School of Hillel, although the Sages believed that both views were valid. Sixteenth-century kabbalist Rabbi Isaac Luria (the Ari) said that not only are both the words of the House of Shammai and the House of Hillel enduring on the conceptual level, but each has its time and place on the pragmatic level as well. In our present world, we follow the rulings of the House of Hillel, but in the era of Messiah, the majority opinion will shift in favor of the House of Shammai, and their rulings will then be implemented. The Ari believed that in our present reality, where divine commandments must be imposed upon an imperfect world, the rulings of the House of Hillel represent the ultimate in conformity to the divine will, while the rulings of the House of Shammai represent an ideal that is too lofty for our present state (which is why we perceive them as stricter and more confining), and can only be realized on the conceptual level. In the era of Messiah, the situation will be reversed: a perfected world will embrace the more exacting application of Torah law expressed by the House of Shammai, while the Hillelian school of interpretation will endure only conceptually.
Hillel's rulings were often based on concern for the welfare of the individual. For example with regard to the remarriage of an aguna, whose husband is not known with certainty to be alive or dead, the view of Hillel (and most of his colleagues) was that she can remarry even on the basis of indirect evidence of the husband's death. Bet Shammai required that witnesses come forth with direct testimony before she was permitted to remarry. Another example of his leniency as compared with Shammai involves converts; Hillel favored the admission of proselytes into Judaism even when they made unreasonable demands, such as one did by demanding that the whole Torah be taught to him quickly "while standing on one foot." Hillel accepted this person as eligible for conversion, whereas Shammai dismissed him as not serious about Judaism.
Sources: Judaism 101
Kolatch, Alfred J. The Second Jewish Book of Why. Jonathan David Publishers, Inc.; Middle Village, New York, 1985.