#3 has given you a pretty good summary. I would only add that Meyer makes some changes to the vampire myth in Twilight. Her vampires are able to come out in the daylight. They can't get into bright sunlight because of something unusual that happens to their skin. Also, her vampires are incredibly beautiful and graceful and exude some kind of fragrance that attracts humans to them. Curiously, they don't have fangs; their teeth are just sharper than humans'.
Twilight is about a normal teenage girl, Bella, who moves to a town where she meets and falls in love with a very mysterious and intriguing boy, Edward... who turns out to be a vampire. This presents a plethora of problems, the largest of which is the fact that her boyfriend struggles with the desire to suck her blood. As their relationship progresses, Bella discovers that there are a lot of secrets out there that she never knew existed, and Edward struggles with the questions of how their relationship might ever work out. Then some antagonists enter the scene, vampires who do not agree with Edward and his family's philosophy of not killing humans. Bella is in danger, and they both have to find ways of keeping each other safe.
This book has become incredibly popular, especially with the recent blockbuster movie.
Check the internet for this...it's a popular book, so there will be lots on it. I've included one link for you: