A format for encoding information in a file. Each different type of
file has a different file format. The file format specifies first
whether the file is a binary or ASCII file, and second, how the
information is organized.
One or several letters at the end of a filename. Filename
extensions usually follow a period (dot) and indicate the type of
information stored in the file. For example, in the filename
EDIT.COM, the extension is COM, which indicates that the file is a
command file. Depending on the operating system, the punctuation
separating the extension from the rest of the filename may or may
not be considered part of the extension itself."