The default is FontFamily. Other common choices are and .
Mathematica will combine settings for FontFamily, FontWeight, FontSlant, FontTracking, and sometimes FontSize to construct a complete name for the font you want. It will then use this name, together with any settings you have specified for and , to try to locate an appropriate font on your particular computer system.
When generating PostScript output on a printer or otherwise, settings you give for are typically used in preference to other font specifications.
Mathematica will try making replacements for the font family name that you specify with the option FontSubstitutions.
Mathematica by default uses heuristics such as translating to for appropriate computer systems.