The following page has some information about OpenType.
In particular, there is a link to the OpenType user guide. A direct link follows.
OpenType is not prescribed in the Unicode Standard, the standard is more general as regards using an advanced font format. There are several such advanced font formats. OpenType is perhaps the most prevalent, yet I think it important to understand that OpenType is a proprietary solution and not necessarily the only advanced font format.
Some years ago I tried to design an advanced font format myself. As far as I know it has never been implemented.
William Overington
28 February 2008