The Sefer Chafetz Chaim (or Chofetz Chaim or Hafetz Hayim) (Hebrew: ?, trans. "Desirer of Life") is the magnum opus of Rabbi Yisrael Meir Kagan, who later became known simply as the "Chofetz Chaim". The book deals with the Jewish ethics and laws of speech, and is considered the authoritative source on the subject.
The title of the Chafetz Chaim is taken from Psalms:
Come, children, hearken to me; I will teach you the fear of the Lord. Who is the man who desires life, who loves days to see goodness? Guard your tongue from evil and your lips from speaking deceitfully. Shun evil and do good, seek peace and pursue it.
The subject of the book is hilchos shmiras halashon (laws of clean speech). Kagan provides copious sources from the Torah, Talmud and Rishonim about the severity of Jewish law on tale-mongering and gossip. Lashon hara, meaning "'evil speech" (or loosely gossip and slander and prohibitions of defamation), is sometimes translated as "prohibitions of slander", but in essence is concerning the prohibitions of saying evil/bad/unpleasant things about a person, whether or not they are true.
The book is divided into three parts:
Rabbi Yisrael Meir Kagan is commonly known as the Chafetz Chaim, the name of his famous work on guarding one's tongue. He was born in Dzyatlava, Grodno Governorate, Russian Empire (today Belarus), on January 26, 1838. As his reputation grew, students from all over Europe flocked to him and by 1869 his house became known as the Radin Yeshiva. Kagan published twenty one books. His first work, Chafetz Chaim (1873), is the first attempt to organize and clarify the laws regarding Lashon Hara. Other notable works include the Sefer Shmirat HaLashon, an ethical work on the importance of guarding one's tongue and the Mishnah Berurah (printed between 1894 and 1907) which is a commentary on the "Orach Chayim", the first section of the Shulchan Aruch, and has been accepted universally among many Ashkenazi Jews as an authoritative source of Halacha. (View the online edition in Hebrew here. Sections 1, 2, 3, 4, 5, & 6)