The most important and
immediately noticeable difference is that the planetary positions are
different. While western astrology uses the actual observed position of the
planets and the zodiac, Vedic astrology uses precession of the equinoxes, and
that's why there will be a difference in the planetary positions, so your
planets may be in an entirely different sign when you use the Vedic astrology
system.
The second important difference is that while western astrology gives a lot of
importance to Sun, Vedic astrology is based on Moon.
In Vedic astrology there is a special tool called the Dasha that is used to
predict the events of a native's life while in western astrology we use
Transits.