Despite The Vampire Diaries TV series being based on the books by L.J. Smith, the storyline and characters are far from it. It has the essence of the books with the soul focus being on the relationship between two brothers with an attraction to the same girl but even Elena, the main character is given a different persona.
Elena Gilbert in the novel is described as having:
fair skin, blonde hair, blue eyes and a wilder than mild personality.
Elena in the TV series on the other hand has the features of actress Nina Dobrev with:
tan skin, brown hair, brown eyes and is given a milder personality.
Katherine in the novel is of Swedish descent but due to Nina Dobrev being of Bulgarian descent, to the advantage of production, so is the character in the series where Nina’s bilingual ability becomes useful in flashbacks featuring Katherine.
-Other Changes-
We see also a change from Elena’s best friend Bonnie, a red headed witch in the novel of druid descent becomes dark haired and of Salem descent due to the casting of Katarina Graham. Casting has had a lot of impact on the changes of character. Not only that but the producers Julie Plec and Kevin Williamson planned to derive the series from the books and yet stretch the plot in their own preferred direction for the benefit of the series, staying recognizably to the book and steering away form it at the same time. Where in the book Elena has a little sister called Margaret, in the series she has a younger brother named Jeremy of high school age. After being signed for his role as Jeremy, Steven R. McQueen recalls reading the book with the intension to learn a bit about his character until he discovered to his surprise that he was replacing a little girl named Margaret.
TVD Interview with Steven R. McQueen
-Changes to the plot-
As far as Season 3 goes, Elena has not yet become a vampire and is saved after crashing off Wickery Bridge. In the novels Elena becomes a vampire after her death over the bridge at the end of the first book in the written series.Another major change is the name of the place in the books from Fell's Church, Virginia to Mystic Falls, Virginia. Personally I prefer the series better but there are a few interesting things about the book such as the vampires who constantly drink blood being able to transform at free will into almost any animal and the Salvatore brothers are from the 1500's rather than the 1800's.