In a computer, a file extension, also called a file format, is the layout of a file -- in terms of how the data within the file is organized. A particular file format is often indicated as part of a file's name by a file name extension (suffix). Conventionally, the extension is separated by a period from the name and contains three or four letters that identify the format.
Application programs must be able to recognize and possibly access data within the file. Sometimes a program may not care whether the file has the appropriate extension name because it can examine the bits in the file to see whether the format (layout) is one it recognizes. Other times, the end user must specify what program should be used to open and work with file.
There are as many different file formats as there are different programs to process the files. A few of the more common file formats are:
- Word documents (.doc)
- Web pages (.html)
- Web page images (.gif and .jpg)
- Adobe Acrobat files (.pdf)
- Executable programs (.exe)