I never ever tell anyone's secrets. This means anything you tell me "don't tell anyone" as well as anything I think you might not want people to know. Or anything that I think you might want people to know, but the person it's about might not want people to know.
I do my best to not talk behind people's backs. I will listen if you feel the need to vent at me about somebody because I know that
a) if you thought someone else would have been better to vent to, you would have gone to them. There's a reason you're talking to me. Either you think I'll understand, you know I won't tell anyone, or I'm the only one around and you really want to get it off your chest. Telling you I don't want to hear it will not help you.
b) if it's really bothering you, you'll probably go to someone else. I don't know that that someone else will keep their mouth shut about it, and if the other person't potential big mouth causes someone else to get hurt, then it's my fault for not providing an ear when someone needed it. I know that what you tell me will never leave my lips. Beyond that, if you choose to tell someone else, that's your fault.
And yes, in a big group of friends when they start making cruel jokes about people I do start doing my whiny goody-two-shoes thing "you guys, stop it! It isn't fair to talk about him while he's not here! and plus you're being mean." And I know they talk about me, too, when I'm not there. But I don't care. I'm not going to be part of group cruelty and cowardice.