Name given to the twelve constellations that the ecliptic crosses - actually it crosses a thirteenth, Ophiuchus, but this is traditionally excluded from the list. Because the ecliptic lies in the general plane of the solar system, the Sun and planets seem to move through the Zodiacal constellations.
A traditional mnemonic:
The Ram, the Bull, the Heavenly Twins, And next' the Crab, the Lion shines, The Virgin and the Scales. The Scorpion, Archer, and the Goat, The Man who holds the Watering Pot, And Fish with glittering scales.
The "tropical" zodiacal dates (those that divide the zodiac starting with the vernal equinox) are used primarily in the United States and western Europe. The "sidereal" dates (those that divide the zodiac starting with the alignment of the Sun with a distant star in Aries) are used everywhere else.
Even people who don't know anything about constellations have heard about the constellations (signs) of the Zodiac in an astrological context.
The astronomers/astrologers (originally the observations and magic/religious applications were made by the same people), tried to use the movements of the night sky with divinatory purposes. Some of these applications where founded on correspondence between practical knowledge and celestial observations (for example, the relationship between solar position and stellar positions depends on the season, which has practical implications for agriculture), some others completely unfounded.
The "sign" under which a person is born, used to depend on the apparent position of the sun relative to the position of the houses of the Zodiac, the constellations on the ecliptic, on his/her birthdate. But because of the precession of the Equinox over the last three thousand years or so, the "signs" are out of phase with the real position of the sun by about a month. Some modern astrologers attempt to take this into account in drawing astrological charts.
The idea of astrological birth sign is that the person would have some characteristics of the arbitrary shape that the ancients identified with a group of stars; so, for example, a "Libra" (the scales) will be balanced and stable. Also, as mentioned in the main article, the planets are found near the ecliptic, and by consequence, in the zodiac. The position of a planet relative to a particular constellation would have a consequence on the life of a person born under not only under that "sign", but also on one with a different birth sign. For example: A person may be born on, say, June 1st. This is near the center of the sign of Gemini, and so Gemini would be his birth sign. Any planets also observed to be within the 30 degrees of sky occupied at its center by Gemini, would be said to have a particularly strong effect on the destiny and personality of the person. If, at the same time, other planets are seen to be within other signs of the zodiac, then their effects would be felt on the portions of a person's life "ruled" by that sign. Significance is also imputed to the angular positions of planets and signs relative to each other at the moment of a birth or other significant event.
Modern astrology tries to dress itself with the garments of real science, using computers and in some cases adding more recently found planets and other objects to make its evaluations and predictions. However, astrology should be regarded as an entertainment (or in the best of cases an art) - one that is followed by millions of people worldwide.