A computer file name has two parts: a base name and a file extension. The two parts are separated by a period. The base name distinguishes the file from other files in the same directory. A file extension tells the computer how to handle the file by following a convention laid down by the creating application's developer.
A file extension is generally a three- or four-letter acronym or abbreviation. An extension can be anywhere from one to 253 characters long, but convention limits its length to three or four characters. An extension can contain letters or numbers, but it cannot contain control characters, periods, slashes, bars, question marks, colons or asterisks.
The file extension allows computer users to quickly identify the nature of a file's contents. Users can group, sort or order files in a directory by file extension. This provides a simple method of keeping all files of the same type together, making searches through lists of files quicker. The duplication of file extensions for different applications, however, means the user has to be aware of possible incompatibilities.
File extensions provide the user with a means of verifying the trustworthiness of a file. For example, an EXE file attached to an email from an unknown sender may indicate that the email is malicious. This ".exe" file extension indicates that the attachment is an executable file, rather than a normal attachment. On the other hand, an email with a TXT file, which only contains plain text, attached is not likely to harm the computer.
Windows uses file extensions to determine how it opens different types of files. When a user double-clicks on a file to open it, Windows will open it with the application associated with that file's extension. The Windows system configuration maintains a list of applications and their associated file extensions. These are called “default programs.” If a particular file extension is registered with a program, Windows will start that program whenever the user elects to open a file with that extension. Only one application, however, can be registered as the default program for each file extension. To use a program other than the default to open a file, right-click the file and choose "Open With."