Bookcase units are pieces of furniture that are designed to hold books. They typically consist of several shelves, which are the main component of the unit. The term 'bookcase units' is often used to refer to the shelves themselves, as they are the most important part of the unit. Shelves are typically made of wood or metal and come in a variety of sizes and shapes. They can be used to store books, magazines, CDs, DVDs, and other items.