The Sun Sign is based on which zodiac sign the sun was in on the date of an individual’s birthday, which is commonly known,  while the Moon Sign is determined by preparing a natal chart based on the exact time and location of birth and noting the Moon's position at that moment.

The Sun Sign refers to a person’s character and ego and how she tends to behave or react to certain situations. The Moon Sign is a good indicator of the emotional self and how one best expresses her emotions, as well as giving quite a bit of insight into how one’s subconscious self works. The combination of these two signs can usually give a fair idea of what someone’s total personality is like.

Since the Sun’s position in the sky can be predicted with a certain degree of accuracy from year to year, it has therefore become the “popular” astrological sign. The Moon, however, changes signs about once every 2-½ days. That makes it a little more difficult to use in forecasting, though not impossible.