There are many different types of files that can be saved on a PC. Each type of file is saved in a specific format that the program that uses that file can understand. For example, an image file is stored differently from the way a text document would be saved.
In windows, each file is saved with what is called a file extension as part of the file name. A file extension is nothing more than a 3 letters appended to the end of a file name after a period (it is usually 3 letters but can be more or less). This file extension is used to identify the file as a certain type so that the computer will know what program to use to open it if you double click on it. For example, if a file is named "sunset.jpg" the computer uses the ".jpg" extension to identify the file as an image file and launches the default program used to view jpg image files. File extensions may or may not show up as part of the file name when browsing with Explorer. This depends on how the file and folder settings are configured.
The actual format in which the data is saved for each file type is independent of the file extension. For example, it is possible to rename "sunset.jpg" so that it has a different extension such as "sunset.mp3". This does not change the file at all. It is still saved in the jpg image file format, but the PC will use an audio player to try to open the file. It will probably not load correctly. Files of a certain type all share the same structure. For example, All jpg files have a similar sequence of bits at the beginning of the file. This information can be used to identify them as jpg files when perform a scan of the data on a disk for deleted file recovery purposes. The first few bytes of 3 jpg files are shown in the figure to the right.