When the spies gave their evil report, they began by saying, "Yes. It is a land flowing with milk and honey and here is its fruit." The Talmud in סוטה says that in order for Lashon Hara to be accepted, it must begin with some truth.
The Talmud in סנהדרין says that when someone gets a reputation as a liar, he's not even believed when he's telling the truth. All of this is to emphasize the importance of guarding our tongue.
Speech can be a very powerful and harmful tool. This is why the Rambam categorizes three types of bad speech. The first is called רכילות, which refers to idle gossip that is true and not necessarily negative. The second type is לשון הרע which is also true but very derogatory. And the third is called מוציא שם רע. This is where one lies about another individual that is most harmful.
The Chofetz Chaim said it best with his simple admonition, "Guard your tongue."