Vedic astrology differs from Western or Tropical astrology mainly in that it uses the fixed zodiac as opposed to the moving zodiac. Because of the gradual tilting of the earth in space on its axis, the zodiac, if you calculate it from the sun's relationship to the earth, appears to be moving at the rate of a little less than 1/60th of a degree per year.
Currently, the relative or movable zodiac is off (out of alignment with) from the fixed or real star-based zodiac by roughly 23 degrees, which is almost one whole sign of the zodiac. Because the two systems are skewed from each other by nearly one whole sign, most people's "Sun Sign"-- that which you can get from the newspaper each day, is usually one sign back when the chart is refigured using Vedic astrology. So, the first surprise of using the Vedic system is that you no longer belong to the Sun Sign you always thought you were. However, if you were born in the last five days or so of the Western sign month, then you will probably still be the same sign in the Vedic system.