In graphical user interfaces (GUI), such as Windows or Mac OS X, a computer folder is just like a physical folder that sits on your desk. This one, however, sits on your computer’s virtual desktop and is used to organize information. It may contain additional folders (which are sometimes called nested folders), documents, or files or a combination of all three. Folders are generally at the top level once you start your computer and access the internal hard drive. Folders may also contain applications or utilities. In DOS and UNIX, folders are called directories.