The default is FontFamily->"Courier". Other common choices are "Times" and "Helvetica".
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 FontPostScriptName and FontNativeName 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 FontPostScriptName 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 "Helvetica" to "Geneva" for appropriate computer systems.