Font File Types
Most modern fonts are stored in either the OpenType or TrueType formats, which can be used by both Macintosh and Windows computers.
.OTF (Open Type)
Font format developed by Adobe and Microsoft; combines aspects of PostScript and TrueType font formats; fully scalable, meaning the font can be resized without losing quality.
The OpenType font format is supported by Mac OS X and Windows 2000 and later.
.TTF (True Type Font)
Font file format created by Apple, but used on both Macintosh and Windows platforms; can be resized to any size without losing quality; also looks the same when printed as it does on the screen.
The TrueType font is the most common font format used by both Mac OS X and Windows platforms.
.dFONT (Data Fork Font: Mac OS)
Standard font format used by Mac OS X; font contents are stored in the data fork part of the file instead of the resource fork (which was used by fonts in Mac OS Classic).
Active system fonts (which can be accessed by all users) must be stored in the/Library/Fonts/ folder. Active user fonts (which can only be accessed by the current user) must be stored in /Users/~username/Library/Fonts/ folder.
DFONT files can be opened by Font Book, or by other Mac OS X programs that access system fonts.
.EOT (Embedded Open Type Font)
OpenType (.OTF) font embedded in a document such as a Web page or PowerPoint presentation (.PPS file); often embedded with Microsoft Web Embedding Fonts Tool (WEFT); saved along with the associated document and not meant to be opened manually.
.WOFF (Web Open Font Format File)
Web font file created in the WOFF (Web Open Font Format) format, an open format used for delivering webpage fonts on the fly; saved as a compressed container and supports TrueType (.TTF) and OpenType (.OTF) fonts; also supports font licensing information.
WOFF files allow Web developers to use custom fonts rather than the standard Web fonts. Many browsers support the WOFF format, but most of them do not support WOFF files until recent versions.
NOTE: WOFF files are referenced within CSS files using the @font-face rule.
*For more info on Font Types, read this article: http://opentype.info/blog/2010/07/31/opentype-myths-explained/