A file extension is the suffix at the end of a filename that tells a computer, and the computer user, which program is needed to open the file. Also called a filename extension, this suffix is preceded by at least one period, and is generally made up of one to five characters, although it is normally three characters in length.
Considering that there are thousands upon thousands of software programs available, and each one has an extension if its own, it wouldn’t be possible to learn each one. People will often find themselves remembering extensions to more commonly used programs, however. Many people find that it is a good idea to familiarize themselves with those used most frequently so that they can accurately recognize the files. Some of the ones that computer users may come across more often are .DOC, which is usually for Microsoft Word® documents, .TXT for text documents, .JPG for JPEG image files, and .EXE for executable files, or files that run a program. When someone comes across an extension that he doesn't recognize, a quick Internet search should be able to reveal its associated software program.
It’s helpful to make sure that a computer shows a file extension name for several reasons. Malicious programs can be sent with executable files with double extensions so that a user is unaware of what type of file he is really dealing with. Opening a nefarious file could expose the computer to a computer virus that could damage the machine and its data.
While it is possible to change a file extension, doing so may cause the file to not open properly, or perhaps even at all. Some programs are able to read different types of extensions, but others may not know how to recognize the change. For this reason, people should be careful when renaming files. They should make sure that they do some research within the program they are using before making any modifications to an extension.