A bookcase is a piece of furniture that holds and stores books. Some bookcases are basically free standing bookshelves. Most bookcases have sides, a top, a bottom and a back with shelves inside that framework. There are some bookshelves that have an unusual shape and they may have bars instead of solid sides and backs, etc. as part of their design. Some are enclosed with doors, especially glass doors, to provide extra protection for the shelves.
To clean the bookcase, just pull over tights around the hoover (vacuum cleaner) and start to gently clean. Another method is to use a duster -- a feather duster or dust rag. You can do a quick dusting of the exposed areas, that is the back of books and exposed shelves, or occasionally, you may want to go through and remove the books, dust the shelves and the books separately.
A bookcase may have a bracket or something attaching it to the wall and keeping it from falling over in case of something like an earthquake.