The Sun and Moon are the life-giving and life-sustaining luminaries, and their sign positions are the most important factors in a horoscope. The blending of their signs results in a very particular and distinct expression, that sets an individual on a different path than someone else with a different Sun-Moon blend.
For instance, Aquarius believes in equality. Someone with the Sun in Aquarius–Oprah Winfrey is an example–can champion causes that promote a more equitable society, and organize their life around it.
While someone with the Moon in Aquarius will make it more personal, by treating people from all walks of life the same way, exhibiting what’s known as the “common touch”. Lady Diana Spencer had the Moon in Aquarius, and was known as the people’s princess:

Winfrey’s Moon sign is in fiery Sagittarius. You could see its expression in her ebullience and warmth, and in her tendency to philosophize on and find meaning in life experiences.
Whereas Lady Di’s Sun is in watery Cancer. She was shy and retiring, which are typical Cancer attributes. Also, as would befit a Cancerian, she known to be a devoted mother to her sons, and beyond that her emotional largesse fed into working with the homeless, troubled youth, and the elderly:

It’s not always easy to separate the expression of the Sun in its sign from that of the Moon, since they tend to be so closely tied to each other. The important thing to remember is that the Moon expression is automatic, we don’t need to even try. But with the Sun sign expression, there is a choice.
Sometimes, the nature of the Sun’s interaction with the other planets blocks its expression. In such cases, there needs to be a conscious attempt to express the Sun’s energies, to feel alive and whole.
Here are a couple more examples of the Sun-Moon blend, and their distinct expression in the life. This first one is of Nancy Pelosi, firebrand speaker of the US House of Representatives:

Compare and contrast Pelosi’s Sun-Moon blend with that of Steve Jobs, visionary founder of Apple:

As with the Winfrey-Spencer combination where Winfrey had the Sun in Aquarius, and Spencer had the Moon there, here too one person, Pelosi, has the Sun in Aries while Jobs has the Moon in that same sign. But Pelosi built her life (Sun in Aries) on the constant thrum of challenge ,whereas Job’s life path (Sun in Pisces) was focused on a visionary quest to deliver the best products possible. Pelosi’s style (Moon in Scorpio) and manner of interaction is one of relentless control, while Jobs’s way of dealing with everyday things was routinely abrasive and combative.