OpenType, a relatively new font format from Adobe and Microsoft, can contain TrueType and PostScript data, and works on both Macs and PCs.
OpenType fonts have the extension ‘.otf’.
Despite the undoubted benefits, such as the inclusion of extended character sets and alternative glyphs (see page 48) in a songle font file, so many pre-OpenType fonts exist that adoption of the new standard can only proceed fairly slowly.
Significantly, though, Apple is supporting it and supplies OpenType fonts with the Mac OS.