In cinema-speak, a logline is basically an elevator pitch. It's a one-sentence summary of plot of the movie you want to write. If you're a screenwriter running around desperately trying to get someone in Hollywood to listen to your story idea, you'd better have a worthwhile (and short) logline that embodies the essence of the story you want to tell.

Here's the one for Die Hard: "A New York cop visits his estranged wife in LA at Christmas time and her office building is taken hostage by terrorists."

Once you've got a logline, should you tell it to anyone? Some people are afraid of sharing their idea, and I'm guessing it's due to the fear that someone might steal it. I think this fear is baseless, since the value behind any idea lies in the details, and the details come only after the execution. I'm pretty sure Hollywood has been stealing loglines from itself for years, with results that end up quite different from one another. Think Freaky Friday and Vice Versa, or Armageddon and Deep Impact.

As far as sharing ideas goes, there's also the fear of rejection. Personally, I like to share things as quickly as possible, to get the rejections out of others quickly, before I sit on an idea for too long and become endowed with a deluded sense of its "greatness".

(P.S. Here's a logline I came up with last month, for a screenplay I might write as soon as I figure out who the characters are: "A millionaire accidentally kills a homeless man with his car, and as the funeral is planned, he's surprised to learn how much he has in common with the deceased.")