The Type42 font is a TrueType font coded in a special way and wrapped in a PostScript font envelope. This can be used to download TrueType fonts to PostScript printers (or PostScript compatible printers) that contain a TrueType rasterizer. This method yields better print quality than can be achieved by converting a TrueType font to a Type1 one.
The complete Type42 font format specification is available at http://vrr.ucw.cz/doc/Type42_Spec.pdf.