Preflight is the process of analyzing native digital files (fonts, graphics, photos) prior to their insertion in a prepress workflow to ensure they are ready for their intended purpose.
Current versions of professional graphics programs have built-in preflight capabilities. Along with these, programs such as Flight Check have the ability to interrogate your files and find hidden problems.
Postflight is the process of analyzing processed files (PostScript, PDF, DCS2) for quality control in a digital prepress workflow. Enfocus Pitstop and Markzware Flightcheck are common applications to do this.