Files, you think you know but what are they really?

Pictures, music, video, spreadsheets, documents and applications are files on your computer. Do you ever wonder what they are and how they differ from each other? What makes a document open in Microsoft Word and a song open in iTunes? Why do files designed to run on Windows PCs like Word documents or Excel spreadsheets also run on Macs and Linux computers? What makes a file an application that a computer can run, instead of displaying lines of computer code?

File names have two parts, the actual file name and the extension. The file name might be something like “Document1” or “IMG_0317”. This is shown to us whether we are working in Windows Explorer or the Mac Finder. The extension, the second part of the complete file name, might be “.doc” or “.jpg” for the two file names above. Both Windows and Macs hide file extensions by default so you may rarely see the “.doc” or “.jpg”. They show application icons instead. But they hide whether that Word file is a “.doc”, “.docx”, or “.dotx”.

I suggest your reconfigure your computer to show the extensions. This is done via XP’s “Folder Options…” or Windows 7/Vista “Folder and search options” in Windows Explorer. Click on the Folder Options “View” tab and uncheck the “Hide extensions for known file types” box. The Mac Finder is easily configured to show file extensions. Go to “Preferences”, click on the “Advanced” tab and check the box labeled “Show all file extensions”.

Now the computer will show both a filename and the extension. This makes it easier to tell what program a data file works with. You no longer need rely upon an icon to tell you. A Word data file usually has the extension “.doc” or “.docx”. An Excel file has an “.xls” or “.xlsx” extension. A photo usually has the extension “.jpg”. Audio files commonly have the extensions “.mp3”, “.m4a”, “.m4p“ or “.wma”.

File extensions are associated with applications. This is why when you click on a data file it opens in its correct program, whether on a Windows PC or a Mac. Typically a Mac computer associates images with iPhoto, audio files with iTunes, Word files with Word or Pages, and spreadsheets with Excel or Numbers. Windows machines may use Windows Live Photo Gallery, Windows Media Player, Word, and Excel as the associated programs, respectively. Adobe PDFs are displayed by default in the Mac Preview program. Most Windows machines use Adobe Reader to open PDFs.

You can open a file in a program other than its default application. Perhaps you want to open an audio file in the VLC media player, a free media player that works on Windows, Mac and Linux computers, but iTunes is your default music playing application (Windows or Mac). If you right-click on the audio file you can choose “Open with” from the Windows Explorer or the Finder context menus. You then select a program from the list or browse for the application you want to use.

Applications (also called programs) are different than data files. They run when clicked rather than launching another program. In the Microsoft world applications typically have the extension “.exe”. Older DOS apps may use “.com”. The Mac application extension is “.app”. So iTunes on Windows is “iTunes.exe” and on Macs “iTunes.app”.

How does the original program code get turned into an application? Programmers use special applications called compilers to transform their program code in to executable applications. Programs issue commands to the computer and its peripherals. A program might tell the printer to print what you are looking at or, if you are in email application, to send the message you wrote.