Expression is used in photography to show the feelings of they person the usually show emotion such as:
> Fear > Anger > Sadness > Happiness > Excitement
Lighting
Lighting is also a key factor in making an portraiture image stand out these lights highlight or shadow certain areas of the face making the image stand out.
In this image light filters have been used to colour the light and highlight certain areas of the models face as well as this it was taken in blue light which focuses our attention to the yellow and pink.