Now we can get to the point: facial expression analysis. 

This concept commonly classifies all facial expressions as relating to one of the six universal emotions—joy (happiness), surprise, disgust, sadness, anger, and fear as well as neutral. 

Emotions are one element that makes us human and (believe it or not) they are difficult to hide, since all emotions, suppressed or not, are likely to have a noticeable physical effect that can be of value if we can automate the process of detecting and then interpreting the physical effects.

Once detected, the interpretation (of facial expressions) process is (perhaps) just another classification exercise. In practice, you'll find that facial expression classification will be based upon what is known as TBM or the transferable belief model framework.