The OpenType font format is an extension of the TrueType font format, allowing support for PostScript font data. Technically there are two OpenType Font flavors; TrueType based (.ttf) and PostScript based (.otf). OpenType was developed jointly by Microsoft and Adobe to produce a hybrid between Type 1 and TrueType fonts, with additional features that work on Macintosh and Windows computers. OpenType fonts can include the OpenType Layout tables, which allow font creators to design better international and high-end typographic fonts.
Some descriptions (of various fields in this document) are copied from the specification available online at:
Use VOLT (Visual OpenType Layout Tool) from Microsoft to add OpenType features to your existing fonts.
More information about VOLT: