Haggai was one of the 12 Minor Prophets in the Torah. He lived in Israel around 539 B.C.E., when Jews were returning from the Babylonian Exile. A short time later, some Jews began to rebuild the Temple in Jerusalem that had been destroyed in 586 B.C.E. The builders became discouraged and stopped working in 520 B.C.E., but Haggai, along with the prophet Zechariah, encouraged the completion of the Temple. Haggai saw the completion of the Temple as the key to Jerusalem's glory and the restoration of the Jewish people in Israel, and assured the builders that the Second Temple would be greater in glory than that of Solomon's Temple. "Who is left among you that saw this house in her first glory? and how do ye see it now? is it not in your eyes in comparison of it as nothing?" (2:3)
Sources: Wikipedia; Telushkin, Joseph. Biblical Literacy: The Most Important People, Events, and Ideas of the Hebrew Bible. NY: William Morrow and Co., 1997.