Gossip is a way of talking about someone behind his or her back, often sharing information that is unconfirmed as true or that is intended to be private, and often with intent to create a negative image of the one being talked about. The Bible speaks strongly against gossip for many reasons. Gossip is not characteristic of God, and nothing good comes from it (Proverbs 13:3).
Some types of gossip are easier to identify than others. Subtle innuendos can mislead someone's thoughts about a person unfairly (Proverbs 26:20). Leading off with something like, "I'm not sure if this is true or not, but apparently…" before sharing some interesting "news" with others can make it seem innocent enough, so we justify our sharing to ourselves. But James 4:17 leaves no room for grey: "So whoever knows the right thing to do and fails to do it, for him it is sin."
If you're unsure if saying something would be gossip or not, it's best to err on the side of caution. We can't "unsay" something we've already said. James 3:5 tells us what a powerful tool our words are. Gossip can be so easy to fall into, but as Christians, it should be something we strive to keep out of our lives and interactions with others, for their and our own benefit. Ephesians 4:29 puts it best: "Let no corrupting talk come out of your mouths, but only such as is good for building up, as fits the occasion, that it may give grace to those who hear."