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File extensions are the letters you see after file names which let you know what sort of file they are.
They also tell your computer which program it needs to open the file.
So next time you receive an e-mail with an attachment like this one, sarah.jpg, you will know it is a photo or drawing because a .jpg file is an image file.
File types
It's useful to know some popular file extensions you may come across online:
| Extension |
Program |
| .doc |
Microsoft Word documents |
| .exe |
program files |
| .gif |
images (used for logos + line drawings online) |
| .htm,.html |
web pages |
| .jpg |
images (used for photos) |
| .mp3 |
Sound + music (used for swapping songs) |
| .pdf |
Adobe Acrobat files (used for saving exact copies of real documents) for printing |
| .png |
Images (a new image format for the web) |
| .ram, .rm |
Real Media sound files (often used for internet radio) |
| .txt |
Notepad file (used for simple text) |
| .xls |
Excel spreadsheets |
If you try to open a file you may sometimes get an error message saying you don't have the correct program for your file.
To find out what program you do need have a look at the extension. If you don't recognise it have a look at whatis.com who claim to have a list of every single file extension in the world!
You can then download the program you need to open your file.
Viruses
One of the reasons knowing file extensions is useful is so you don't open files which could damage your computer.
People don't worry about going to web pages or opening up .jpg files that friends e-mail them but think twice before opening up a word document or excel spreadsheet.
That's because .htm (web pages) and .jpg (image) files are data files. Your computer does not execute them which means that it doesn't give them permission to do anything other than be looked at.
Other files like .doc (Word documents) and .xls (Excel documents) are able to run programs and that means that they can damage your computer.
Of course, a good virus checker should keep you safe but it's always worth thinking twice before opening any file on your computer which can execute.
And, if you don't know who sent it - keep clear!
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