One of the most basic body-language signals is when a person

crosses his arms across the chest. This could mean that a person
is putting up an unconscious barrier between themselves and
others. But this can also indicate that the person's arms are cold,
which would be clarified by rubbing or huddling the arms.
Tapping toes, swinging feet or drumming fingers can mean that person are bored of something. Also person could show his boredness by the signals of tiredness like yawning. His whole body may sag as he slouch down in his seat.

Anger people usually are baring of teeth and snarling.
Sad people we can noticed by trembling lip or tears.
Raised eyebrows, widening of eyes and opened mouth shows that a person is surprised.
It is very simple to recognise happy person by his beautiful and shining smile.
Body language it is very important because it could say much more about people than words.