DESCRIPTION is one of the four chief types of composition (along with argumentation, exposition and narration) that has as its purpose the picturing of a scene or setting. Although sometimes used for its own sake (as in Poe's Landor's Cottage) it more often is subordinated to one of the other types of writing, especially to narration, with which it most frequently goes hand in hand.
If you write YA, go easy on the DESCRIPTION. Younger readers love dialogue and will avoid a book that has large paragraph blocks describing the environment. Keep your descriptive passages brief and include in the dialogue tags to make your story a fast read.
Some genres are known for fuller use of description, such as fantasy where the readers need to visualize the world created in the story. And literary fiction. In fact, in these particular genres, if the author doesn't utilize DESCRIPTION properly, their readers will feel cheated.
(This post has been inspired by and in some instances, directly quoted from A Handbook to Literature, 8th Edition, by William Harmon and C. Hugh Holman)