A filename extension is a suffix to a filename. There’s the name of the file itself, then a period, then the extension. When you see Document.pdf, the ".pdf" is the extension. The filename extension is one of those things that’s a necessary evil in computing to differentiate one type of file from another.
Some UNIX/Linux environments purposely use specific extensions for the sake of convenience because they’re technically not required. For example, the Debian distribution uses files that end with .deb to represent a Debian software package. When you see a file that ends with .deb, you instantly know what it is. You can consider filename extension use in a UNIX/Linux environment to be more of a descriptor (i.e. metadata) than anything else.
The Windows environment requires filename extensions to operate. Traditional extensions do not exceed three characters. For those of you thinking, "Isn’t is true all file types in Windows use three-character filename extensions?" Not necessarily. 7-Zip for example uses the two-character .7z by default, and several programming languages use one or two-character extensions.
Common filename extensions that confuse people as to what they are
Some extensions are really easy to figure out because they’re part of a word.
Examples: TXT (TeXT), DOC (DOCument), BAT (BATch script file), COM (COMmand), WAV (WAVe sound file), SH (SHell)
Then there are those that are abbreviations, like PDF (Portable Document Format).
But then there are those that do nothing but confuse people because they don’t know what they are. Here’s a few should you encounter them:
M3U and PLS
These are both file types that represent a multimedia playlist. Many get confused when they encounter these because they think there’s actual audio within them. Not true. They are small "pointers" to other audio files (such as MP3) either local to your computer and/or on the internet.
The super-long title for M3U is "Moving Picture Experts Group Audio Layer 3 Uniform Resource Locator". Try and say that 5 times fast.
PLS to the best of my knowledge means either PLaylistS or PLay Stream, although I could be mistaken (and if I am, feel free to correct me by posting a comment).
ODT
This is a file type that that is slowly becoming recognized in mainstream use, that being the OpenDocument Format. The T is for text. There is also ODS with S for spreadsheet, ODP with P for presentation, ODG with G for graphics and ODF with F for formulae (mathematical equations).
Most people know ODT files from using the freely available OpenOffice suite. The ODT format by the way is compatible with Google Docs if you don’t have software to read them.
RAR
Most people know what this is but there are still a few that don’t. It’s a Roshal ARchive. This is an archive format like ZIP. If you want to open one, use 7-Zip or WinZIP (paid software). If you want to create one, use WinRAR (also paid software).
FLV
FLash Video. If you want to view an FLV video independently (meaning not in your web browser), use the freely available VLC.
FLAC
Free Lossless Audio Codec; an audio file type that sounds better than MP3 because it is lossless while MP3 isn’t. To play them you can use Windows Media Player or the freely available WinAMP.
3GP and 3G2
Both of these are usually video file formats and part of the 3GPP standard. You will usually only encounter these when someone sends you a video file via a cell phone. Windows Vista and 7 will view these via Windows Media Player. If using XP, use Apple QuickTime.
M4A and MP4
Both are MPEG-4 Part 14.
Both file types can be either audio or video. While it’s true that most files ending in .m4a are audio-only, it doesn’t mean they have to be.
Windows Media Player 12 – which is bundled with Windows 7 – will play this format. For older versions of Windows you can use Apple QuickTime or the freely available VLC.
OGG
A newer multimedia file format. It can be audio or video. This format can be opened and played on many different programs, including the freely available VLC.
Older file types you may encounter
What’s listed below isn’t in mainstream use any longer, but should you encounter them you’ll at least know what they are and how to deal with them.
ARJ
An archive file type like ZIP and RAR.
ARJ is Archived by Robert Jung. This file type was actually quite popular during the 1990s and at one point gave ZIP some really good competition. 7-Zip is able to open these.
WPS
This is a proprietary document format used by the Microsoft Works when you save a document with it. It is the file extension everybody hates because nobody can open them that doesn’t have Works.
Fortunately you can do a Save as in Works and save in DOC format, but most people who use Works do not do that. And if you use Works, I strongly urge that you always save in DOC and not WPS – especially if you’re sending documents to other people.
Got a bunch of WPSes and need to convert them? Use Zamzar to do it. (WPS conversion, thankfully, is supported.)
TIF or TIFF
This is an image file type. It stands for Tagged Image File Format. Most image editors can easily open TIFF files, including the Windows Picture and Fax Viewer which is included with the Windows OS.
If for whatever reason you have a problem (which you shouldn’t) opening a TIFF, use the freely available GIMP software.
RTF
This is Rich Text Format, a document type developed by Microsoft way back in 1987. It is still, even in Windows 7, used as the default file type when saving from it’s bundled word processing application WordPad.
For older generation computer users, RTF sticks out like a sore thumb due to the fact so many people used Windows Write (the predecessor to WordPad). But many newer generation users have never seen RTF files.
Shades of the past: Using the Start/Run dialog in Windows 7, if you type write and click OK, WordPad will load up.
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