The grapevine is a network of friends - colleagues - whatever - that passes information informallay as in gossip. "I heard it through the grapevine that you were thinking about taking a job with another company." It's not official. I heard it from some friends that we have perhaps. Here's a video of a famous song with that idiom. It also has the lyrics / text with it. http://www.youtube.com/watch?v=hajBdDM2qdg

I heard through the grapevine that she was pregnant, but I don't know anything more.

it means to hear news from someone who heard that news from someone else. cowboy...

The grapevine is a good image to describe how news can travel quickly, but also twist & turn in the process. There was a term created after the telegraph was invented in the US: "grapevine telegraph", describing the speedy but sometimes inaccurate method of spreading news via word-of-mouth. The Australian term is "bush telegraph". And now I'm singing the Marvin Gaye song in my head...

It means that you may have heard some information about work related matters or gossip about people's lives through unofficial channels, like at a party in a restaurant etc.