OpenType is a format for scalable computer fonts. It was built on its predecessor TrueType, retaining TrueType’s basic structure and adding many intricate data structures for prescribing typographic behavior.
The specification germinated at Microsoft, with Adobe Systems also contributing by the time of the public announcement in 1996. The specification continues to be developed actively and is migrating to an open format.
Based on Unicode, an OpenType file can contain up to 65,535 characters or glyphs. This allows for extensive language support and makes room for advanced typographic features like ligatures, various figure styles, fractions, stylistic alternates, swashes, small caps, ornaments, borders, and so on. All these extras can live in one file instead of many. A single OpenType file contains all the information required for a typeface style: metrics, kerning, outline, hints, and bitmaps.
OpenType is a truly cross-platform format. You can use the same font on Mac or Windows machines without converting the font or fearing that reflow. Everywhere you use an OpenType font, it is the same typeface, same kerning, same line breaks.
Because of wide availability and typographic flexibility, including provisions for handling the diverse behaviors of all the world’s writing systems, OpenType fonts are used commonly today on the major computer platforms.