Extension
The extension is the second part of the name of the file.
It is often three characters long, but can be longer. The
extension tells the computer which program, or application,
to associate the file
with.
For example, if you create a document
in Microsoft Word, the computer automatically assigns that
file the extension "doc". That code tells the computer that
it is an MS Word document,
and should be opened using Word.
The first part of the file
is the filename
