What it is
Jyotish is the Indian system of astrology. It reads the positions the grahas actually held at the minute and the place you were born — nine points, measured along the ecliptic, charted from where you were standing.
How it differs
Western astrology is generally tropical: its signs are anchored to the equinoxes, and so to the seasons. Jyotish is generally sidereal: its signs are anchored to the constellations themselves. The two frames have drifted roughly 24° apart since they agreed. Same birth, a different chart — for most people, the sun sign moves back by one.
Why it tells you when
A life is divided into planetary periods of fixed length — a Vimshottari cycle runs 120 years, and no two periods are the same size. Knowing which one is running is the difference between a description of your character and a read on the stretch of life you are standing in.
And what is moving now
Transits are the live sky against your fixed one. Divisional charts narrow the same birth moment to a single department — work, marriage, money. Together they answer when, not just what.