Parshat Behar discusses the issue of "ona'ah" - harming others. In Leviticus 25:14, the Torah prohibits harming others financially, for example by charging inflated prices. Then in verse 17, the Torah prohibits harming others with words.
Some people would find this impractical, saying that "you can't legislate morality." But that only applies in a non-God system. When we are conscious of God watching over us, then even interpersonal relations have a self-enforced standard of right and wrong. That's why verse 17 concludes with the words, "and you shall respect God."
The Talmud discusses exactly what is included in this prohibition of "harming others with words."
One idea is that we shouldn't remind someone of his negative past. Let's say that Joe was a wild bachelor, who has now settled down into a respectable family man. He has worked hard to put the wild days behind him, and it would be embarrassing - even harmful - for us to recount stories of his escapades.
Acting in a misleading way is another aspect of "harming others with words." Let's say you're not looking to buy a new computer, but are just curious about what new models are available. So you go into the computer store and begin asking a bunch of questions. The salesperson, of course, thinks you're interested in buying, and as the conversation continues - about features and prices - the salesperson builds up hope that you will actually buy.
There is an unstated assumption that you entered the store to buy. Your words are, in effect, misleading the salesperson, even unintentionally. In such a case, the Torah would permit you to satisfy your curiosity in the computer store - providing that you make it clear from the start that you "are only looking, with no intention to buy."
At first glance, we might think that harming others financially is more serious than harming others with words. In fact, the opposite is true. A person's property is only peripheral to him, yet his feelings are an essential part of who he is. Being sensitive to another's feelings is, in the eyes of Torah, a great mitzvah, that we should always strive to fulfill.