Great Portraits capture personality, mood, energy, a moment in time.
They focus on the face or head and shoulders. Generally the person is aware of being photographed, but not necessarily
posing.
Candid Photographs capture spontaneity, a person's enjoyment of an activity, life.
The people photographed act naturally, ignoring or unaware of being photographed.
Sometimes the photos capture motion.
Some expressions only last a fraction of a second... and some never repeat.