In Mac OS and Mac OS X, what are file types and creators?

In Mac OS 9.x and earlier, every file has a type and creator code, which indicate the file type and associated application. Each type and creator is a unique four-character code that identifies a kind of file or specific application. Based on this information, the Finder determines the icon to assign to a document, and which application to open when you double-click it.

Although type and creator codes are supported in Mac OS X, native OS X applications do not use them for new files. Instead, OS X relies on MIME types, filename extensions, and since Mac OS X 10.4, Uniform Type Identifiers.

Also see:

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