Richelle Mead has created an intriguing world where there are 'good' vampires- The Moroi and 'bad' vampires- The Strigoi. A lot of this is based around actual Romanian mythology.
The story is told by Rose Hathaway. Rose is a Dhampir, which means she is half Moroi vampire, half human. Dhampirs are stronger and faster than Moroi who are unusually delicate for vampires. Dhampirs also do not drink blood. The Dhampirs are supposed to protect the Moroi from their mortal enemy the Strigoi.
Strigoi are essentially the traditional vampire. They are created whereas Moroi are born. Moroi can become Strigoi by draining and killing somebody. Moroi drink blood but only in small amounts from consenting humans known as 'feeders'.
Rose's best friend Lissa is a Moroi princess and Rose sees it as her duty to protect her. Once they finish school Rose hopes to become Lissa's guardian- a trained Dhampir assigned to protect a member of Moroi royalty.
Dhampirs are trained as guardians because they are faster and stronger than Moroi and can protect them from Strigoi. Guardians are taught that only Moroi life matters and they protect them with their own lives.
Lissa and Rose are especially close, and as the series progresses we start to understand why. There is remarkable continuity throughout the whole series with each book developing the story a little more, keeping with the same themes and building on them.
Rose is a strong heroine- literally physically strong, and her character really develops throughout the series. I have to admit that I would have liked to have seen more development in 'Last Sacrifice' and I felt that she didn't really grow as a person in this book.
My absolute favourite book of the series was the 4th- Blood Promise. I think I enjoyed it most as it was where we saw Rose grow the most as a person, and it had a real sadness to the whole story.
I have seen comparisons made to Twilight, but I think that they are very different. Yes there are vampires, yes there's a love story but the worlds they are set in and the characters are really so very different. Fragile, worrying Bella is a totally different heroine than rebellious Rose with her killer left hook.
The only real similarity (vampires aside) for me is that Vampire Academy is another series that I just couldn't put down.