This is a single line that describes the work, defining it in some way, and is usually used in advertising a film.
For example, in the classic movie Alien, the tagline was “In space, no one can hear you scream.” The line was used for advertising on the movie poster, and it incorporates three elements: “In space” lets the audience know that this is a futuristic setting in space. “No one can hear you” emphasizes the idea that the protagonist is utterly alone, beyond the reach of help, and the word “scream” tells us that this is horror.
Here are a few more taglines from movies I grabbed from Wikipedia:
“Who you gonna call?” – Ghostbusters
“There can be only one.” – Highlander
“One ring to rule them all.” – The Lord of the Rings
“Love means never having to say you’re sorry” – Love Story
“To boldly go where no man has gone before” – Star Trek
“The truth is out there.” – The X-Files
“Why So Serious?” – The Dark Knight
“Just when you thought it was safe to go back in the water…” – Jaws 2
“Nothing on earth could come between them.” – Titanic
“Trapped in time. Surrounded by evil. Low on gas.” – Army of Darkness
“Some houses are born bad.” -The Haunting
“It knows what scares you.” -Poltergeist
“You will believe.” -Ghost
So a tagline is used for advertising a film or book, but what else is it good for?
As a writer, your tagline can be considered a touchstone. It may define certain elements of your story, things that you dare not vary. For example, let’s say that you chose that Alien tagline—“In space no one can hear you scream.” You begin writing, and as you’re halfway through the screenplay you decide that your heroine, Ripley, really needs a love life. So you create a nice male protagonist whom you decide will survive to the end of the story. Maybe she’ll save him from the aliens, or maybe he’ll save her.
If you’re a wise writer, you will realize that “Hey, I just violated my promise to the audience. I wrote a tagline that emphasized loneliness.” Well, if that’s the goal, you’ve got only one choice: the aliens have got to kill that love interest.
Sometimes a tagline will come to you only at the end of a novel or screenplay, but at others a tagline will hit you like a bolt from the blue, almost demanding that you go out and write a tale. So learn to love them, and use them. Begin paying attention to them now, critiquing them, so that you learn to craft them well.