The Oxford Paperback Dictionary defines the zodiac as "a band of the sky containing the paths of the sun, moon, and principal planets, divided in twelve equal parts (called signs of the zodiac) each named from a constellation that was formerly situated in it." At the center of this band is the plane of the ecliptic defined by the relative motion of the earth and sun. The width of the band allows for the fact that the orbits of the other bodies are inclined relative to plane of the ecliptic. The zodiacal year begins at the point where the plane of the ecliptic intersects with the earth's equatorial plane at the vernal equinox when the sun moves into the northern hemisphere of the earth's equatorial plane. Although the signs derive their names from the constelations, they are not the same thing. For example, although the sun always enters the sign of Aries at the vernal equinox about March 20, it will not cross into the astronomical constellation of the same name until nearly a month later.
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 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 for divinatory purposes. Some of these applications were founded on correspondences 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), while some others were completely unfounded.
The familiar "sign", more precisely the "sun sign", under which a person is born usually refers to the apparent position of the sun in the signs on the tropical ecliptic at the time of his or her birth. Because of the precession of the equinoxes 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, and do not correspond with the astronomical constellation in which the sun then finds itself. Some modern astrologers attempt to take this into account in drawing horoscopes.
The idea of astrological birth sign is that the person would have some characteristics of the mythic symbolism that the ancients identified with that constellation; so, for example, a "Libra" (the scales) will be balanced and stable. Also, since the planets are all found in the zodiac. The position of the moon or a planet in a particular sign would have an effect on the life of that person. For example: A person may be born on June 1st. This is near the center of the sign of Gemini, and so Gemini would be his sun sign. Any planets also observed near the center of Gemini, would be in "conjunction" with the sun, and said to have a particularly strong effect on the destiny and personality of the person. At the same time, other planets are in other signs of the zodiac, and their effects would be felt on the portions of a person's life "ruled" by that sign. Significance is also associated to the angular positions of planets and signs relative to each other at the moment of a birth or other significant event.