I think when listening isn't met with encouragement and edification, and/or you're the 3rd in "3rd party", then it becomes gossip.
Well said Redeemed86.
And if I may add, never ever gossip or entertain gossip. When I first became a Christian, I never forget a lesson I learned from a young girl at Church. I was standing talking to her one day after Church Service, and a man approached us and started telling us how bad another person was in Church and tearing down that person. The young girl stayed silent for a few seconds, and said, "Please do not gossip to me about others". Simple as that, and the man walked off. And I have never forgotten that day and that was 12 years ago.
Gossiping usually involves tearing others down, and the person is spreading their poison to try to turn everyone else against that person. We should avoid such discussions and tell the gossiper that it's not appropriate.
If someone is truly evil and going around hurting people and are dangerous, sure it may be appropriate to warn others, but talk about SOLUTIONS to help the person and try to help edify them, not tear them down and spread more poison.
That's how I feel about it anyway, and I could never see Jesus standing around backstabbing and gossiping about others, so neither will I.
God bless.