In astrology as well as astronomy, the Zodiac is a 360° circle around the earth, where each of the twelve Zodiac signs is 30°. In astronomy it is simply used as a coordinate system, to mark the positons of celestial bodies. In astrology, though, it is much more.
     In astrology and the horoscope, each Zodiac sign represents a character, a tendency, which affects any celestial body therein. The planets express what happens, the Houses where it happens, the aspects why, and the Zodiac signs how.
     Every planet moves through the Zodiac, and changes character according to what sign it is in. The planets are the acting forces in the horo­scope, but the Zodiac signs nuance and alter their character.
     Here are the twelve Zodiac signs, their English mea­nings, and keywords for their respective characters:

    Aries, the Ram   challenging, impulsive
    Taurus, the Bull   conservative, concrete
    Gemini, the Twins   communicative, playful
    Cancer, the Crab   caring, sensitive
    Leo, the Lion   dominant, proud
    Virgo, the Virgin   careful, minute
    Libra, the Scales   negotiating, ethical
    Scorpio, the Scorpion   hidden, passionate
    Sagittarius, the Archer   impatient, independent
    Capricorn, the Goat   ambitious, decisive
    Aquarius, the Water Bearer   profund, original
    Pisces, the Fishes   searching, submissive

Each sign can also be said to have a motto, an expression that helps to clarify its character. Here are the mottos of the twelve Zodiac signs:

    Aries, the Ram   "I am."
    Taurus, the Bull   "I have."
    Gemini, the Twins   "I think."
    Cancer, the Crab   "I feel."
    Leo, the Lion   "I want."
    Virgo, the Virgin   "I scrutinize."
    Libra, the Scales   "I balance."
    Scorpio, the Scorpion   "I wish."
    Sagittarius, the Archer   "I see."
    Capricorn, the Goat   "I use."
    Aquarius, the Water Bearer   "I know."
    Pisces, the Fishes   "I believe."

     The characters of the Zodiac signs are to a large extent dependent on their element and quality. Every sign belongs to one of the four Greek elements, which represent four aspects of life:

1. Fire   activity
2. Earth   materialism
3. Air   thought
4. Water   emotions

     Since there are twelve signs and four elements, there are three signs belonging to each element. There are also three different basic qualities that signs have:

1. Cardinal   leading
2. Fixed   static
3. Mutable   following

     So, each sign has its own combination of element and quality:

     In principle, the combination of element and quality that is unique to each sign, decides what its character is.
     Each Zodiac sign is also said to have a ruling and an exalting planet. This simply means that some pla­nets are particularly enforced by the character of that sign. A planet rules over the Zodiac sign where it fits better than the other planets do, and it exalts in a sign where it fits better than any other planet than the ruler of that sign.
     Here are the ruling and exalting planets (note that also the sun and the moon are regarded as planets in this context) of the Zodiac:

     Since there are twelve Zodiac signs and only ten planets to divide between them, Venus and Mercury both rule and exalt in two signs each.
     Regarding what planets rule and exalt in what signs, astrologers don't agree completely, but most of the above is generally accepted. In any case, the rulership and exaltation should not be overly emphasized in horo­scope interpretations. A planet's character in a sign speaks well for itself, without these generalizations.
     When people are said to be born in a certain Zodiac sign, it only means that the sun was in that sign at their birth. It's their sun sign. The sun is only one component in the horoscope - although an important one - so pre­dicitons based only on that are not astrologically ade­quate. The whole horoscope has to be considered.
     The sun moves steadily through the Zodiac signs, spending about 30 days in each of them. The calendar system of leap years makes the dates of change from one sign to another jump around a little, but in average, these are the dates that the sun spends in each sign:

    Aries   March 21 - April 19
    Taurus   April 20 - May 20
    Gemini   May 21 - June 20
    Cancer   June 21 - July 22
    Leo   July 23 - August 22
    Virgo   August 23 - September 22
    Libra   September 23 - October 22
    Scorpio   October 23 - November 21
    Sagittarius   November 22 - December 21
    Capricorn   December 22 - January 19
    Aquarius   January 20 - February 19
    Pisces   February 20 - March 20

     If you are born on a date of change, or the day before or after it, you should have your sun position calculated by a horoscope program, to ascertain what sun sign you are.
     The traditional order of the Zodiac signs starts with Aries, because it is the moment of the spring equinox. That date was also the date of the new year, in ancient Europe. At the fall equinox, the sun moves into Libra. At the summer solstice it enters Cancer, and at the winter solstice Capricorn.
     The symbols used for the Zodiac signs are the same in astrology and astronomy. I have made ink brush ver­sions of those symbols, just for fun. Here is a more strict rendering of them: