What Are Signatures?
The Macintosh doesn't use three-byte extensions (or even longer than three bytes, nowadays) to identify the very nature or contents of files, but signatures.
Signatures are strings of eight bytes, four for the creator (the program which created the file) and four for the file type (text, picture, and so on).
See also our Comments for Techies.
The correspondance between signatures and icons is managed by the Finder, for all programs which happened to exist on a volume, in the Desktop file (an hidden system file which is never shown by the Macintosh but exists on every disk). With System 7 and upwards, the Desktop graduated to a double Desktop DB + Desktop DF, for reasons which would led us too far away from our main issue.

Signatures Tables
Here are some of the signatures and corresponding PC extensions we collected (if you want to add or correct something, please feel free to send us an e-mail, we try to update the site frequently).
For those who need to edit the internal table of our Mac-PC transfer programs (MacDisk and MacImage), you will have to note the signature/extension you want to use/exploit and put it in the table displayed by the SignEdit utility (see the manual). If you encounter an atypical file (something not giving the results expected), please check the real signature of a file whose origin doesn't make any doubt, on a Macintosh magnetic volume (see the rightmost column of the display).

Generic Formats
These signatures identify the file type, but not the creator ('????' is a valid creator). They are useful when a program refuses to load a file because the signature is not what it expects and the programmer was too lazzy to look into the file. We give the PC extension when there is a well established one.

Specific Formats
These signatures identify both the file type and the creator. Applications other than the creator generally only look at the file type to ascertain whether they can open the file. (The '_' represents a space character.)

Below a more general list of filetype and creator strings we already saw (the '_' represents a space character).

File Types

Creator Strings
Please note that '_' stays for a space.

More Info for Techies
Signatures are often considered as strings, and the greatest part of information published on signatures is based on this assumption.
However, signatures are handled by programs as 32-bit integers, which means that when you see a filetype coded as 'TEXT', a program will see 0x54455854, or even 0x54584554, depending on whether it is big-endian or little-endian.
This is the explanation of the possible presence of spaces (very frequent) or even of cryptic characters, even of non-printable characters, in the signatures.
Please note also that Apple reserves for its own use all creator strings made only of lower case letters.
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Where are Signatures Stored?
These signatures, against what is often mentionned, even by knowledgeable persons, are not saved in the resource fork of each file, but in the catalog tree (on the disk, in a management structure you will [hopefully] never have to look at).
This mistake finds its origin in the fact that the programs (and only the programs, not the data files) put the icons they use in their resource fork and have to declare (in a bundle resource placed in their resource fork) the signature of the files they intend to manage, so that the Finder knows what to do when the user double-clicks on an icon. Another source of errors comes from the fact that some graphic files save in the resource fork a preview picture of the file (thumbnail to display on the desktop).

Other Resources
Two other sites may be helpful:

• Apple signature registering page
  Signatures [or at least creator strings] should be registered with Apple. This site allows you to consult the database and know whether a string is free or registered. There is no possibility to get a exhaustive list (and Apple stated publicly that there would be no such list).
• Ilan Szekely's Index
  Index to download, containing more than 30 000 signatures.