"You shall not go around as a gossipmonger"—Leviticus.

~*~

"Evil gossip kills three: the one who says it, the one who listens, and the subject of the gossip"—Talmud.

~*~

So you know something that Mr. A has done. Non-incriminating, not even objectionable. Unless you have a compelling reason, you are forbidden to share this information. Repeating innocuous gossip is called rechilut—and often causes unforeseen negative consequences.
Speaking about another's indiscretion or shortcoming is even worse; this is called lashon hara (the evil tongue). Unfounded libelous gossip (motzee shem ra) is even worse.
Someone trying to sell you some juicy information? Politely excuse yourself or change the subject. Words carry the potential of causing catastrophic harm, often tearing asunder families and friendships. Thankfully, lashon hara awareness has increased in past decades, largely influenced by the passionate writings of the Chafetz Chaim (Rabbi Israel Meir Kagan) on the topic.

1. It is also forbidden to listen to lashon hara. Someone trying to sell you some juicy information? Politely excuse yourself or change the subject. Better yet, explain why you are not interested in listening.
2. Sometimes, even a "compliment" can have a negative connotation. Example: "My neighbor is a great chef! The aroma of barbeque steak wafts into my yard every night!" Is this also a veiled critique of a spendthrift lifestyle?
3. "Oh, don't ask; I'd rather not talk about Mark..." Lashon hara wasn't said—but it was implied!

Note: We are obligated to notify a person of another's conspiracy against him. We are also required to share information with any person in a position to help the offending person. For example, you certainly should inform parents if their child is hanging out with the wrong crowd.

The Power of the Tongue
Exerting self-control over our speech is admirable. Even greater is the ability to truly respect and love every person, automatically eradicating negative, and losing the desire to share negative information.
The destructive power of negative speech is only surpassed by the beneficial power of positive speech. Praising and speaking positively about our fellows benefits ourselves, the person being praised, and all of society.