Twilight is an ultra-popular saga consisting of 4 books by Stephanie Meyer. Mainly appreciated by girls from age 8-16, the series has made billions of dollars in films and other merchandise. However, no matter how "great" the fans make it seem, the writing is actually quite poor and the plot is boring and unoriginal. The actual definition of "saga" (a word commonly used to describe the series) is "a long story of heroic achievement", which is definitely NOT what Twilight is. The protagonist is a plain girl named Bella with no personality or talent (besides being incredibly boring and uninteresting). She ends up falling in love with a vampire, Edward (who is also quite boring and one-dimensional). Many of the fans of Twilight adore this character, saying he's 'hot'.
These books set bad examples for society. For instance, Bella is willing to DIE than not be with Edward. This is emphasizing the idea that having a boyfriend is the most important thing in life, and that women should be protected like material.
The series is often compared to another ultra-popular series, Harry Potter
by J.K. Rowling. In contrast to Twilight, these books are great. Go read them.