OpenType is a font format. A font is a container for all letters and characters ("glyphs") of a particular typeface. Each glyph is stored in forms of scalable curves (outlines). OpenType fonts are based on the same file structure as the TrueType font format but with some new elements.
Originally started by Apple, then developed by Microsoft and Adobe. In a way, it unites the PostScript and TrueType font formats in one, although there still are two distinct "flavors" of OpenType: OpenType PS (OpenType fonts with PostScript outlines) and OpenType TT (OpenType fonts with TrueType outlines). The name is spelled "OpenType", not "Open Type". "OpenType" is a registered trademark of Microsoft Corporation. That's the origin.
An important element of OpenType is the OpenType Layout -- a set of advanced typographic features that enable easy automatic access to small caps, fractions, ligatures, swashes etc.
OpenType TT fonts are fully backwards-compatible with TrueType fonts. OpenType PS fonts need more recent operating systems or newest ATM installed. Normally, OpenType fonts are compatible with all printers and imaging devices. Some very old PostScript devices are not compatible with OpenType TT fonts but they are compatible with OpenType PS fonts.
For more information, please refer to:
1. Start with this:
http://store.adobe.com/type/opentype/CA-Aug02.pdf
2. Then this:
http://store.adobe.com/type/opentype/
3. Then this:
http://www.myfonts.com/info/opentype/
4. Then this:
http://store.adobe.com/type/browser/OTReadMe.html
5. And finally this:
http://www.microsoft.com/OpenType/
Regards,
Adam Twardoch