OpenType fonts come in two so-called flavors, PS (PostScript) and TT (TrueType).
You may have heard that there are two kinds of OpenType font: OpenType PS flavor and OpenType TT flavor. One of the “big ideas” of OpenType was to put an end to the competition of font formats, meaning it should no longer be an issue which format font you buy.
There are a couple of reasons why this isn’t quite true.
OpenType terminology redefines Windows TrueType fonts as “the TrueType (TT) flavor of OpenType”, sometimes called OpenType TT and having a “.ttf” file suffix. Microsoft and other font producers including Linotype typically produce OpenType fonts in the OpenType TT flavor. Since OpenType TT fonts are for all practical purposes internally the same format as Windows TrueType fonts, there are no problems getting such fonts to work in Windows. Even Windows 3.1 from 1992 will display such fonts.
In addition, we have the PostScript (PS) “flavor”, sometimes known as OpenType PS or CFF-based OpenType and having an “.otf” file suffix. Adobe and font producers including Elsner+Flake, Emigre, Storm and URW++ typically produce fonts in the OpenType PS flavor.
Current versions of both Windows and Mac OS X support both OpenType flavors. Almost all applications running on Windows XP/Vista and Mac OS X are able to use both OpenType flavors, although some of the functionality in the font may not be available. Please refer to the article OpenType support in systems and applications for a more detailed view.
To sum up at this point, both OpenType TT and OpenType PS fonts can be used now for your projects, even on equipment and applications that are several years old. But before purchase, check whether your intended applications support the features in the flavor of OpenType you wish to buy.