A lot of the actual luminaries' attributes can be used to describe their meaning in astrology:
The Sun makes life on earth possible with it's light and warmth and can be seen during the day.
The Moon reflects the Sun's light and can be seen at night, it controls the tides of the ocean.
Together, they create the dynamic of night&day, light&darkness, shining&reflecting.
Those are the two major sides to your personality. To describe it with Jungian Psychology, it is like your Self and your Shadow.
Sun/Self: The force that gives you the vital energy to take action, your light of consciousness, how you act rather than react, your identity, ego and self-presentation.
Moon/Shadow: Your reflective, unconscious, emotional world and your attitudes as well as habits. How you react rather than act, what drives you and motivates you to take actions.
The Moon describes what you want, how you think, what motivates you and the Sun describes how you then get active to do the things to get that. Sun is controlled, conscious action that is motivated by unconscious, emotional needs and wants of the Moon.
What you do consciously is just the tip of the iceberg.
Unconscious, habitual reactions and attitudes outweigh conscious actions.
Thats why, in some schools of astrology, the moon sign is considered to be more important than the sun sign.
Moon is who we are, Sun is what we do.
We live by the Sun, we feel by the Moon.
Sun/Moon Combos:Leo & Sagittarius
Libra & Aquarius
Scorpio & Aquarius