The document describes the Facial Action Coding System (FACS), which is a method for comprehensively measuring and describing all observable facial movements and expressions. FACS breaks down facial expressions into their constituent Action Units, which concentrate on areas like the eyebrows, lips, cheeks, and forehead. Each Action Unit describes a specific facial muscle movement, like lowered eyebrows indicating annoyance or raised eyebrows showing surprise. FACS allows for detailed analysis and identification of facial expressions.