The protagonist of this story is Elena Gilbert, a high school senior as the first novel begins, living in Virginia. She is beautiful and popular and happy, though perhaps a little bit shallow. Here is a description from the first book:
Elena Gilbert, cool and blond and slender, the fashion trendsetter, the high school senior, the girl every boy wanted and every girl wanted to be.
She is a somewhat of an archetype - the popular cheerleader with the football player boyfriend. That is Matt Honeycutt, a nice guy who unfortunately loses out on this pretty girl.
What causes Matt to lose Elena is the entry of the book's leading man and secondary protagonist, Stefan Salvatore. He is a new boy at school, but the bigger story is that he is a vampire and he is tormented by his past. His relationship with Elena will eventually be a source of salvation for him.
As far as antagonists go, enter Stefan's older brother, Damon Salvatore. He isn't a true villian, but his arrogance and his impulsiveness cause many problems, particularly as he tries to steal Elena's affections from Stefan - just as he did hundreds of years before.
Which leads to the next character, Katherine, a beautiful and seductive vampire that the vampire brothers knew and were involved with in Renaissance Italy. They believe she committed suicide, but this turns out not to be true and she reenters their life in the present.
Klaus is a true villian - he is the vampire that created Katherine and who brings evil and strange occurrences to the small Virginia town.
Bonnie McCullough is Elena's best friend and confidante, and has her own supernatural powers - she is a psychic. Her foil is another friend of Elena's, Meredith Sulez, a level-headed and practical young woman who counteracts the passions of many of the other characters.
Caroline Forbes is another potential antagonist, but of the high school world as opposed to the supernatural world. She is a pretty and manipulative rival of Elena's.