The Sun sign represents your basic essence and vitality. It's how you experience your ego, and the traits in your Sun sign speak to qualities you're learning to embody.
The Moon sign represents your emotional nature and the feeling instinct. It's how you experience nourishment and safety. It's what you need to feel fulfilled.
The Ascendant is what I like to call the "cover of your book." It reveals the qualities you most visibly show to others upon first impression. While this cover is definitely not the entire story, it does provide significant information about how you approach your world.
It's very easy to put together some sentences with keywords from each sign that allows you to uncover anyone's cosmic trinity. I'll use myself as an example to illustrate the simplicity.
My Sun is in Taurus, my Moon is in Sagittarius and I have Aries Rising. Without knowing anything else about my chart, you can surmise the following.
It's that simple! Now, following the same format, I encourage you to do this for yourself and anyone else you're curious about. Find out your own Sun, Moon and Rising signs -- or anyone else's!