OpenType is a computer font format that was built on its predecessor TrueType, intended to supersede both the TrueType and the PostScript Type 1 font formats.
The outlines in an OpenType font may be defined in TrueType format outlines, or Compact Font Format (CFF) outlines based on the PostScript language.
For most purposes, it doesn’t matter what the outline data format is because applications support both formats equally well. But for some purposes, such as rendering on screen, TrueType outlines are superior if they contain the hinting instructions that improve legibility at low resolutions.
The term ‘OpenType’ by itself doesn't specify outline data format, instead terms like ‘OpenType (PostScript flavour)", ‘OpenType PS’, ‘OpenType Type 1’, ‘OpenType CFF’, ‘OpenType (TrueType flavour)’, or ‘OpenType TT’ are used to indicate which outline format a particular OpenType font contains.
Most of fonts that Typotheque offers for licensing are in OpenType CFF format. Only Typotheque Screen fonts use OpenType TrueType outline for improved rendering at small sizes.