Star Rating System:
From Goodreads: Young-adult fiction (often abbreviated as YA) is fiction written for, published for, or marketed to adolescents and young adults, roughly ages 14 to 21. Young-adult fiction, whether in the form of novels or short stories, has distinct attributes that distinguish it from the other age categories of fiction. The vast majority of YA stories portray an adolescent as the protagonist, rather than an adult or a child. The subject matter and story lines are typically consistent with the age and experience of the main character, but beyond that YA stories span the entire spectrum of fiction genres. The settings of YA stories are limited only by the imagination and skill of the author. Themes in YA stories often focus on the challenges of youth, so much so that the entire age category is sometimes referred to as problem novels or coming of age novel. Writing styles of YA stories range widely, from the richness of literary style to the clarity and speed of the unobtrusive. Despite its unique characteristics, YA shares the fundamental elements of fiction with other stories: character, plot, setting, theme, and style.
From Goodreads: Adult fiction, by contrast with children's or young adult, is fiction intended for (and usually about) adults.
Extra: I also read some books that I think are rather beyond the YA rating. These are usually books from the genre of Erotica. I try to be as general public friendly as possible and I won't usually feature them on my blog but if I do, I will give fair warning for any unsuitable content.