From earliest times home computer files had a three character tag attached to file name, e.g. letter.txt. The .txt tag is the file extension.

Why do we have this?  It identifies the file type - .txt is a text file. To further the filing cabinet analogy of computer files to paper system this would be similar to colour coding filing folders or tagging letters to indicate the type of information they contain. Extents cause your operating system to take certain actions - a .com file (command) or an .exe (execute) file are programs, other codes are used by your word processor or email program etc.

The extension that is associated with your file is determined by the programmer who writes the code. File types may be 'standard' like .jpg, a picture format.

How many extensions are there? Just use Goggle and search 'file extensions'.

If you do not see 'extents' in windows explorer on your computer, it is because Windows has already "registered" it and Windows default does not show the extent for any registered types. To see the extents go to Tool>Folder Options> View and 'unclick' the "hide extensions for known file types" button.  Remember to 'apply' the change. From then on you will see the extents associated with your files. On the 'view' button click 'details' to see the file sizes with the names.

If you encounter a file type that windows does not recognize, it will ask you what program you want to use to view your data.

Shortcuts and Icons.

A shortcut is simply a way to run a program without going through the:
start> programs> manufactures name> program selection. Instead you simply click on the shortcut. The shortcut is designated by an icon. You can create a shortcut by right clicking on an executable program then clicking on the 'create shortcut' bar. The resultant shortcut can be dragged to the desktop.

If you don't like the little picture (icon) associated with your new shortcut - you can change it by right clicking on the icon, then properties, and then change icon. You can then select from a display of available icons for your use or make your own. There are programs that allow you to make icons but its easy to use a program like Irfanview. Select your favourite picture and save it somewhere as type .ico. Go through the change process, select browse and find your own icon and install it.