The Facial Action Coding System (FACS) was first described by Paul Ekman and colleague Wallace V. Friesen in their article entitled Measuring Facial Movement for Environmental Psychology and Nonverbal Behavior1(1) fall 1976.
The authors’ goal “… was to develop a comprehensive system which could distinguish all possible visually distinguishable facial movements.” The system presents a series of Action Units (or ‘AU’s in the FACS terminology) which can be used to taxonomize human expressions.
See, for example, a video of : AU2 the Cheek Raiser and Lid Compressor. (For copyright reasons Improbable is not permitted to reproduce the video here, but you can access it (AU6s.mpg) via this page)
Researchers can purchase the entire FACS system on CD-ROM here for US$260 (plus shipping).