What a Vedic chart shows
A Vedic chart, or Jyotish chart, is the sky at the moment you were born, read the way Indian astrology reads it. It uses the same planets in the same places in the sky as a Western birth chart, but measures them in the sidereal zodiac, counts the houses as whole signs from the ascendant, and adds two pieces the Western chart doesn't have: the 27 nakshatras, or lunar mansions, and the dashas, the planetary periods that time a life.
The calculator gives the three placements a Vedic astrologer asks for first: the lagna (the rising sign), the rashi (the Moon's sign) and the nakshatra of the Moon. Then it draws the chart in both traditional layouts, lists every planet with its sidereal position, nakshatra and house, and works out your Vimshottari dasha periods from birth to 120 years.
Sidereal and tropical zodiacs
Western astrology uses the tropical zodiac, which starts at the March equinox. Vedic astrology uses the sidereal zodiac, which is fixed to the stars. The two drift apart by about 1° every 72 years as the Earth's axis slowly turns, and the gap between them, called the ayanamsa, is now about 24°. That is why a Vedic chart usually puts your Sun, Moon and planets one sign earlier than a Western chart: someone born with the Sun at 10° Capricorn has it at about 16° Sagittarius in the Vedic chart.
Neither zodiac is the wrong one; they answer different questions and come from different traditions. The birth chart calculator shows the same moment in the tropical zodiac.
Lagna, rashi and nakshatra
The lagna is the sign rising on the eastern horizon at birth. It sets the first house, and every other house follows sign by sign, so it needs an accurate birth time. The rashi is the Moon's sign; many Vedic astrologers treat it as your sign, and it is often the sign used for daily and yearly forecasts in India. The nakshatra refines it: the Moon's position among the 27 lunar mansions, with its pada (quarter) and ruling planet. Your nakshatra is the starting point of the dasha cycle.
North and South Indian charts
Vedic charts are drawn as squares, not wheels. In the North Indian style the houses stay in place, with the first house (the lagna) at the top center, and each house is labeled with the number of the sign in it (1 for Aries through 12 for Pisces). In the South Indian style the signs stay in place, with Pisces at the top left and the signs running clockwise, and the lagna's sign is marked. Both show exactly the same chart; people use the one they grew up with.
The planets and the nodes
Vedic astrology reads nine grahas: the Sun (Surya), Moon (Chandra), Mars (Mangala), Mercury (Budha), Jupiter (Guru), Venus (Shukra), Saturn (Shani), and the two lunar nodes, Rahu (north) and Ketu (south). Uranus, Neptune and Pluto are not part of the traditional system and are left out. Rahu and Ketu are always exactly opposite each other; the chart uses the mean nodes, as most Vedic software does.
Vimshottari dasha
The dasha system divides life into planetary periods. The Vimshottari cycle lasts 120 years: Ketu 7, Venus 20, Sun 6, Moon 10, Mars 7, Rahu 18, Jupiter 16, Saturn 19 and Mercury 17. It starts with the planet that rules the Moon's nakshatra at birth, part-way through that planet's period (as far as the Moon had traveled through the nakshatra), and then runs in that fixed order. Each period divides into nine sub-periods, the antardashas, in the same order starting with its own planet. The calculator marks the period and sub-period you are in now.
How the positions are calculated
The planets come from the same astronomy engine as every chart on this site, which is checked against NASA JPL Horizons. The Lahiri ayanamsa is computed the way the Swiss Ephemeris computes it (its value for the year 2000, carried by the IAU 2006 precession, plus nutation), and the site's tests compare the whole Vedic chart with the Swiss Ephemeris in Lahiri mode at forty dates from 1900 to 2099: the ayanamsa agrees within a second of arc, and the planets, the nodes and the ascendant within half a minute of arc (about a hundredth of a degree), far finer than a pada. For future dates the fast-moving Moon can differ by up to a minute of arc by 2099, because nobody yet knows exactly how the Earth's rotation will slow. Dasha dates use a 365.25-day year.