There are many books out in the world, and those books are separated into categories. Then those categories are further divided. The fiction genre is one example of a first category, and it is then divided into realistic fiction, historic fiction, science fiction, fantasy, etc. Lately, I’ve noticed the blurring of the line between science fiction and fantasy. So, I will point out the differences once again.
Science fiction is mostly comprised of books about what would happen should technology advance, or certain things happen to the earth that are not planned. As these things are both unlikely to happen and their results hard to predict, most of the effects are guesswork by the authors. One book in which technology advances is Uglies by Scott Westerfeld, in which the government completely controls the minds of the people, and make everybody look the same when they reach a certain age. However, there are other societies in the wilderness where escapees live like we live when we go camping. One girl travels there, but only to spy on them. It has a good amount of betrayal, and at the end the plot is gripping and your eyes skim over the paragraphs to find out what happens next.
Fantasy is more of a genre where the ideas are completely made up. Usually it involves magic, mystical creatures such as vampires, or religion. Twilight by Stephenie Meyer is an example of fantasy, although mostly it is a romance. Personally I found the style it was written in unoriginal, but the star-crossed lovers plot line was very gripping and kept me reading it, no matter how much I disliked the characters.
A better example of traditional fantasy for teens would be the book Once a Witch, which is by Carolyn MacCullough. It is about how a magic-less teenager struggles to find herself in a family where magic is everything. The characters were sweet, and the bad guy was both easy to hate and easy to understand.
I hope you go on to read all of the books though, as both of the genres are amazing.