File Types
On your hard disc, information is kept in files. There are basically two types of files - programmes and data.
Programmes are software instructions for the computer to do things. Examples include the Firefox browser, Irfanview image viewer, Excel spreadsheet programme.
Data files are what these programmes work on. Examples include a web page, email message, Word document, picture image.
Microsoft Windows determines the file type from the filename extension. This is the last 3 letters after the dot. Thus, orange.doc is a Word document, and orange.jpg is a picture. Other operating systems use different ways of determining file type but the filename extension is still important.
Examples of programme file types
| .bat | batch file |
| .com | executable |
| .dll | dynamic link library |
| .exe | executable |
| .scr | screen saver |
| .vbs | visual basic script |
Examples of data file types
| .bmp | bitmap picture |
| .doc | Word document |
| .gif | picture |
| .html .htm | web page (hyper text markup language) |
| .jpg | picture |
| portable document format | |
| .png | picture |
| .txt | plain text document |
| .xls | Excel spreadsheet |
| .zip | compressed file |
This is by no means a complete list - there are thousands of extensions.
When you click or double click on a file, your computer looks at the filename extension to work out what to do with it. The extension is the last three letters after the dot. In general, it will run programme files and will open data files.
If malware can pretend to look like a data file to you, you might be tricked into double clicking it, intending to open it but actually running a programme (the virus). Windows often hides the filename extension, making it difficult for users to avoid this trap. Thus AnnaKourikova.jpg.exe looks like AnnaKourikova.jpg.