A file is the common storage unit in a computer. All programs and data are contained in a file, and the computer reads and writes files. A folder holds one or more files, and it can be empty with just a name. Folders provide a method for organizing files similar to a manila file folder containing paper documents in a file cabinet. Folders are also called "directories," and they are created on the hard disk when the operating system and applications are installed. For example, the user's desktop is actually a folder. File Extensions Files are identified by a short "extension" at the end of their name. For example, ABC.JPG is a JPEG image, ABC.DOC is a Microsoft Word document file, and ABC.EXE is an executable application in Windows. Sometimes Files Are Really Folders Although not identified as such, what appears to be a single file may really be a folder. For example, starting with Microsoft Word 2007, the default document format was no longer a DOC file, but a DOCX file; in reality, a ZIP file containing folders and files (see Office Open XML and ZIP file). The same change occurred in Microsoft Excel 2007, migrating from XLS to XLSX formats for worksheet files. In the Mac, an application has an APP extension, and what appears to be a single file is actually a folder (see APP file). See file, folder and extension.