Dfont or data fork TrueType is a special type of TrueType font format, specifically developed for Macs running Mac OS X. Some of the system fonts that ship with OS X are dfont fonts. You won’t find this file format on any other operating system.
For more information on TrueType fonts, check this page. In essence, TrueType is a popular font format on Macs, Windows, and Linux machines. TrueType fonts look good on-screen and offer excellent output quality. TrueType fonts have been in use since the early 1990s and are now gradually being replaced by the more recent OpenType font format.
In earlier Macintosh operating systems such as System 9, files could store data in two entities called the resource fork and the data fork. For TrueType fonts this mechanism was also used: all font-related information was stored in the resource fork. With OS X Apple decided to move away from the concept of forks. Files now only have a data part. This means that the old System 9 TrueType fonts were no longer usable in OS X. To avoid having to renegotiate a font licensing deal for its system fonts, Apple decided to move the data from the resource fork of System 9 TrueType fonts into the regular data section of OS X files. Thus we got dfonts or data fork TrueType fonts.
When people tell you to avoid using Dfont fonts within professional publishing environments, this advice should be taken with a grain of salt. Dfont fonts have the same qualities as other types of TrueType fonts. It is only a pity that they are not cross-platform compatible.
Dfonts on Macs running OS X
Some of the OS X system fonts provided by Apple are dfonts. These include Courier, Helvetica, Monaco, Symbol, Times, and ZapfDingbats. Dfont fonts have the file extension .dfont.
The problem with dFonts is that there can be naming conflicts with legacy PostScript or TrueType fonts. In the case of duplicate font names, the dFont fonts should be removed to avoid text rewrap when using the incorrect font type. For prepress usage, it is common practice to use the PostScript versions of Helvetica and Times. In OS X versions up to 10.4 replacing the dfont fonts by PostScript versions was no issue. With OS X 10.5 things are a lot more complicated due to its system font protection mechanism.
Dfonts on Macs running System 9
Earlier on this page, I mentioned that dfonts are unique for OS X. This is true but for compatibility reasons, it can be useful to convert Dfonts to regular TrueType fonts or back. A utility called dfontifier does exactly that. It can be useful if you still share documents with OS 9 users.
Dfont fonts on Windows or Linux systems
No other operating system except OS X supports dfont fonts. Windows or Linux systems cannot display dfont fonts.
When you copy a dfont file to a Windows or Unix machine, it retains it file size whereas regular Mac TrueType fonts become 0K. When the dfont file is copied back to an OS X system, the font can once again be used because its data structure is still intact.