In the Science of Astrology there are two Schools of thoughts. One is the Western School of thought that we all are pretty familiar with and the other one is the Vedic School of thought. Now this Vedic School of thought, though pretty similar to the Western one is very different. Two questions that come to mind immediately are: Why is it different? And how is it different?
The first thing is that the Vedic School uses a constellational way to measure the zodiac; that is, it uses the actual constellations in the sky to mark out the signs of the zodiac. Fundamentally it is more astronomically accurate and is called the sidereal zodiac. The Western School uses the tropical zodiac, which determines the boundaries of the twelve signs by marking out exact thirty degree sections of the sky starting from the vernal equinox; that is, the exact position of the Sun on the first day of spring.
Second point is that Sidereal astrology is constellationally based, whereas Tropical (Western Astrology) is seasonally based. It is not to say that one system is better than another. You could also say that the Vedic-sidereal system is based on the relationship between the earth and the stars, whereas the Western-tropical system is based on the relationship between the Earth and the Sun.
Now to address the issues of interpretation. The Vedic School describes your acts, your job, family, health and other situations very accurately. The Western School, on the other hand, describes your mind and its workings. It has a more psychological interpretation. To sum up, Vedic Astrology describes what you do and Western Astrology describes what you think.
A lot of people ask how it is possible for a person to possess two different charts. This is because in the Vedic you might be an Aquarius, but in the Western you are a Capricorn. The question that arises now is which one is right? The only answer one can give is that these are two different interpretation styles based on two different measuring systems. Theoretically though, they are equal to each other.
It can be argued that the Vedic system is more accurate simply because it is a system that is relatively intact. The Vedic tradition is from India where spiritual disciplines have not changed in a thousand years.
Western culture however has suffered tremendous losses though the ravages of the Dark Ages in Europe, where literacy was rare and open minded spiritual inquiry was stamped out by the Church for a thousand years for the fear of being displaced by another religion. Western astrology carries the baggage of the materialistic "intellectual" culture and is crude compared to the complexity and subtle refinement of the Vedic astrological system.