Do not go about as a talebearer among your people.
Vayikra (Leviticus) 19:16
The repercussions of the mitzvah of proper speech, and the transgression of Lashon Hara (Evil Tongue), are so intense that they have literally shaped the destiny of our people. The Jewish People have been in exile for 2,000 years, because of words that come out of our mouths.
The power we wield when we speak is far beyond what we can perceive. A key factor in our relationship to G-d and in living our lives as Jews. Lashon Hara is so powerful that it can erase the merits of a lifetime of Torah learning and mitzvah observance.
Just as the negative consequences of speech can be so vast, the positive consequences of proper speech are even greater. The Vilna Gaon says that proper speech is the single biggest factor in determining one's portion in the World to Come. The Chofetz Chaim tells us that adherence to the Laws of proper speech empowers our davening, validates our Torah learning, accesses G-d's Divine Protection and invokes the many blessings G-d, in His kindness, is waiting to shower upon us.
When a person speaks or listens to Lashon Hara, 31 mitzvot may be violated. Even though one does not generally violate them all at once, it is important to remember how carelessness can lead one into deeper trouble. This applies equally to Rechilut and Lashon Hara.
Rechilut is often the repeating of Lashon Hara. For example, Reuven tells Shimon that Levi is ugly (Reuven spoke Lashon Hara), and then Shimon tells Levi what Reuven said about him. Shimon probably made Levi angry with Reuven, which is Rechilut.
The Torah delineates different situations and conditions, and identifies when the speech is forbidden, permissible, and even desirable. One type of Lashon Hara, speaking lies (slander) is called "Motzi Shem Ra" (spreading a bad name). It's pretty easy to imagine how lies, and even exaggeration, can unfairly damage someone's reputation.
A case discussed in the Talmud involves someone who has purchased an item at a "no exchanges, no returns" market. The Talmud instructs us to say that it's a nice buy, regardless of what it is in reality. This case shows that truth is not always the deciding factor in ethical Jewish speech. In fact, the definition of Lashon Hara does not reflect truth or falsehood at all, but the damage that it can inflict.
But sometimes we speak Lashon Hara because we forget that in many cases, truth can be subjective (like "beauty is in the eye of the beholder") or elusive, in that we don't always know the whole picture.
In righteousness shall you judge your kinsman.
Vayikra (Leviticus) 19:15
This verse commands us to give the benefit of the doubt. We are expected to always judge other Jews favorably, to believe that there may have been factors of which we are not aware. Don't judge other people, unless you find yourself in their situation. As you judge others, you will also be judged. In other words: "Think twice before you speak (or judge)."