The main prohibition against lashon hara is derived from Leviticus 19:16 : "Thou shalt not go up and down as a talebearer among thy people; neither shalt thou stand idly by the blood of thy neighbour: I am the LORD.". The Talmud (tractate Erchin 15b) lists lashon hara as one of the causes of the Biblical malady of tzaraath. In Sotah 42a, the Talmud states that habitual speakers of lashon hara are not tolerated in God's presence. Similar strong denouncements can be found in various places in Jewish literature.
There are times when a person is obligated to speak out, even though the information is disparaging. Specifically, if a person’s intent in sharing the negative information is for a to’elet, a positive, constructive, and beneficial purpose, the prohibition against lashon hara does not apply. Motzi shem ra, spouting lies and spreading disinformation, is always prohibited. And if the lashon hara serves as a warning against the possibility of future harm, such communication is not only permissible, but, under certain conditions, compulsory.
"Lashon" is translated as "language" or "tongue". The word is generally translated as "evil speech". It is true that the concept of lashon hara is regarding true and correct statements. Lies and false and exaggerated information fall into a worse category called Hotzaat Diba, or derogatory/slanderous or defamatory speech which is, in fact, worse than lashon hara in many ways.
The two major halakhic works on lashon hara are Chafetz Chayim and Shmirat HaLashon ("guarding [of] the tongue") both by Rabbi Yisrael Meir Kagan (1873). Chafetz Chaim lists 31 speech-related mitzvot mentioned in the Torah. The English book Guard Your Tongue anthologizes the teachings of these two books and provides many examples of prohibited speech.