What are Ruling Planets?:
Each of the planets has rulership over one or two of the Zodiac signs.
How were Ruling Planets assigned to the Zodiac Signs?:
Based on their observations over time of qualities and "energies" of both the planets and the Zodiac signs, early astrologers started to match them up.
When were they first assigned?:
Some have remained unchanged since the earliest days of Astrology, while other pairings were made after the discovery of Uranus in 1781, Neptune in 1846 and Pluto in 1930. The discovery of Chiron in 1977 and the recent demotion of Pluto to dwarf planet may alter these associations further.
What does it mean that a planet is "Ruler" of a particular sign?:
It means that the Ruling Planet, and all its energies, are in charge and is the primary influence. For example, if your Sun is in Gemini, the influence of Mercury prevails as far as your Sun sign is concerned. The "energy" of the Sun is harnessed by Mercury, whose influence overrules the Sun's unique qualities, and determine the way that solar power is expressed.
The Planets and their Zodiac Sign Rulership:
The Moon rules Cancer.
Mercury rules Gemini and Virgo(traditional ruler).
Venus rules Taurus and Libra.
Mars rules Aries and Scorpio (traditional ruler).
Jupiter rules Sagittarius and Pisces (traditional ruler).
Chiron rules Virgo (modern ruler)
Saturn rules Capricorn.
Uranus rules Aquarius.
Neptune rules Pisces.
Pluto rules Scorpio (modern ruler).