In honor of Twilight Saga: Eclipse's release in less than 15 hours (not that I'm counting or anything) I am going tell you my opinions on each of the books. :O)
In Twilight, we meet Bella Swan as she is making the move from Phoenix, Arizona to dank, rainy Forks, Washington. From this first act, we learn that Bella tends to put others before herself-- she moved for her mother. She moves in with her semi-emotionally distant father, but it works because she's not the best at expressing her emotions either. She is instantly the most exciting thing to hit Forks High simply because she's new (evidently, they don't get new kids often).
Bella notices the Cullen kids and instantly seems to aggravate Edward Cullen to the point he can't stand to be in the same Biology class with her. Eventually, Bella learns Edwards big secret after he saves her from 4 attempted rapist in the neighboring Port Angeles, Washington. Instead of being freaked out that Edward and his family are vampires, Bella is drawn to the family and starts a dating relationship with Edward. Of course being good friends with a family of vampires leads to its own set of troubles when a group of nomads interrupt a baseball game. These red-eyed nomads don't partake in the same "vegetarian" diet the Cullen approve of and the protective stances the family take around Bella causes a serious complication.
I absolutely love the Twilight saga! I think one of the reasons I like the series so much (even though I usually hate all things Vampire) is because the vampire/supernatural part of the books is not really the central focus. The books are more about family, friendship, first love and lasting love. The Vampire thing is more of a secondary story plot and not the main thing.