Q. This is a stupid question so please don't use my name.

What do all the letters mean at the end of a computer file? Sometimes I see XLS, sometimes PDF and other strange combinations - after the period.

A. This is actually a very good question. You should never be ashamed about asking a question about computers - PCs are a very broad and complicated topic that nobody has mastered completely.

When a computer file is saved to your hard disk, so you can use it again later, it usually has a name already or you are prompted to give it a name. You normally want to make the name something that will make sense to you when you read the name weeks, months or years from now.

For example, "my 2004 tax information" is a good, descriptive name that you can recall as needed. "Taxes" is not as good a name because you will probably have many years of tax information stored and this could be confusing.

Even worse is a generic name like "Important" or "Files."

Once you have a good name for the file and save it in the program you created it in, the program will probably add a period and then 3 (sometimes 4) letters to the end of the name.

So if you created a document in Microsoft Word and saved it as "2004taxes", the file would be saved as "2004taxes.doc" The letters added to the end of the name are called the filename extension and are very useful.

When you open a file that you created or that was sent to you via e-mail or you grabbed from the web, etc. the computer will load the program that corresponds to the extension of the filename.

So if I sent you an e-mail attachment called "Mytaxes2004.doc", when you open that file, the computer will launch Microsoft Word (if you have it installed) and open the document.

Windows keeps track of file extension and the preferred program to launch when it encounters that extension. (NOTE: this is a very good reason NOT to automatically open attachments you get in e-mail. Programs can launch to cause havoc with your system)

You may have noticed at some time that a certain program used to launch when you clicked on a certain type file, but now a different program launches. That is because often when you install a new program, it wants to rule everything and it changes your settings and makes itself the preferred program for any files it can handle.

You can view a list of what programs are associated with what extensions by opening Windows Explorer and selecting the Tools Menu and then Folder Options and choosing the File Types tab.

Here are some common file extensions you should be aware of:

Txt

Plain text files - none of the fancy formatting of a word processing document. Notepad or a word processor is usually used to read it.

Htm or html

A web page. Your web browser (like Internet explorer) opens these files

Doc

A word processing document - Word, WordPerfect and other such programs use this.

Xls

An Excel spreadsheet file

Pdf

Adobe created a format that allows formatted documents to be read as they were created even if the user doesn't have that program on their PC. You need the free Adobe reader program to open PDF files.

Exe

this is a program file, an executable. Be very very careful before opening a file with an exe extension. It will launch the program when you click on it.

Zip

Some files are large so they need to be compressed to be manageable. Or they consist of several files that need to stay together. In those cases, a file might have a zip extension indicating they were compressed and combined with a utility like PKZIP or WinZIP. Those same utilities will normally be needed to extract the files.

Gif

A graphic file - very common format for pictures on the Internet.

Jpg

Another popular format for pictures - usually for photos and more complex images than gif.

Tif or Tiff

Often a scanner saves scanned images in this format. Tif files are usually very large and very high-resolution.

Mp3

A very popular format for CD-quality sounds. Napster and the music downloading services use mp3 files.

Wav

Windows native sound format. If you record something into your Windows Sound recorder, it will be saved a s a wav file

Avi

A Windows format for video files

Mov or QT

Movies created in QuickTime (usually Macintosh) will have one of these extensions.

Mpg

Like mp3 sound files, but for movie files.

There are a lot more, but this should get you started. Ask if you need help identifying any others.

Answered by Tech Expert Dan Hanson