The Macintosh Operating System uses TYPES and CREATORS. Here is an overview of what each one does.
TYPES: The desktop file keeps track of the types of the files. Typical types are MW2D (MacWrite II), FNDR (the Finder), FMPR (FileMaker Pro), APPL (applications) and TEXT.
When you open a file from within an application, the application looks at the type of each file to see if it can be opened. (Most applications work this way. Some applications look into the header of the file to find out what they are.)
CREATORS: The desktop also keeps track of the creators of files. As you might guess, this is a code which describes which program actually created the file. Typical creators are FMPR (FileMaker Pro), MPNT (MacPaint 2.0) and ERIK (the Finder).
As an example, a text file saved out of MacWrite II would have a type of TEXT and a creator of MWII.
You can find out the type and creator of a file using a utility like ResEdit, DiskTools, Norton Utilities or DiskTop. You can also change the creator and type of a file using these utilities, but this can be dangerous and should only be recommended to advanced users and with the understanding that we cannot be responsible for damage they may do to the files.
FORMATS: A format is a proprietary set of rules and instructions that are used to write files that can be read by certain applications. An example of a format would be PICT or Claris CAD files, or FileMaker Pro files. The DXF format in AutoCAD is an easy one to take a look at, since DXF files are text files and you can open them in a word processor.
When an application reads in a file it expects certain information, in a certain order. Usually there is a header, where global information about the particular file is kept, such as the pen widths or fonts used in the file. Then there might be a database of objects if it is a drawing program, or some text if it is a word processor.
When you translate a file via translators such as XTND, or the Claris Graphics Translator, those utilities change the format of the file. As an example, if you use a utility such as ResEdit to change the type of a MacPaint file to MDPR then try to open it in MacDraw Pro, MacDraw Pro will begin to open it then give an error message. Since the file will still have the format of a MacPaint file, MacDraw Pro will not be able to open it.
Formats for given applications are never used by the ordinary user, but may be needed by developers who are writing translators or other utilities.