You gotta watch it, catch it, read it. And don't be an MCP guys...it's a great story.
Twilight

Twilight is an action-packed, modern-day love story between a teenage girl and a vampire. Bella Swan (Kristen Stewart) has always been a little bit different, never caring about fitting in with the trendy girls at her Phoenix high school. When her mother re-marries and sends Bella to live with her father in the rainy little town of Forks, Washington, she doesn't expect much of anything to change. Then she meets the mysterious and dazzlingly beautiful Edward Cullen (Robert Pattinson), a boy unlike any she's ever met. Edward is a vampire, but he doesn't have fangs and his family is unique in that they choose not to drink human blood. Intelligent and witty, Edward sees straight into Bella's soul. Soon, they are swept up in a passionate, thrilling and unorthodox romance. To Edward, Bella is what he has waited 90 years for a soul mate. But the closer they get, the more Edward must struggle to resist the primal pull of her scent, which could send him into an uncontrollable frenzy.

If you could live forever, what do you live for?

Twilight:
"The apple on the cover of Twilight represents "forbidden fruit." I used the scripture from Genesis (located just after the table of contents) because I loved the phrase "the fruit of the knowledge of good and evil." Isn't this exactly what Bella ends up with? A working knowledge of what good is, and what evil is. The nice thing about the apple is it has so many symbolic roots. You've got the apple in Snow White, one bite and you're frozen forever in a state of not-quite-death... Then you have Paris and the golden apple in Greek mythology—look how much trouble that started. Apples are quite the versatile fruit. In the end, I love the beautiful simplicity of the picture. To me it says: choice."

New Moon:
"[The flower on the cover] is a ruffled tulip. As for the meaning... If you've read the Twilight FAQ, you know that the apple cover had a lot of meaning for me, and I was an active part of the covering process. However, that experience is more the exception than the rule in the publishing world. Something to keep in mind if you intend to embark on a career as a writer: lots of things you might expect to be under your control are not. Covers, for example. Those are mostly up to the publisher and the marketing and sales departments. So I don't know what the tulip means—I didn't have anything to do with this one."

Eclipse: same as New Moon- Stephanie didn't have a role in the cover.
[There are however, 2 main theories:

1. The ribbon represents Bella tearing apart as she chooses between Jacob and Edward / tries to find which is her true love.

2. The ribbon has to do with how fragile compromises, such as Bella's between Edward and Jacob, are, and shows that compromises cannot always hold the pretty ribbon together.]

Breaking Dawn:
"Breaking Dawn's cover is a metaphor for Bella's progression throughout the entire saga. She began as the weakest (at least physically, when compared to vampires and werewolves) player on the board: the pawn. She ended as the strongest: the queen. In the end, it's Bella that brings about the win for the Cullens."