Because I have so much to do and because this subject is well-known, and the Wikipedia page well written, rather than re-invent the wheel I'm quoting here from Wikipedia's "Planets in astrology". When I have more time I'll fill in the corresponding archetypes and equivalences with other symbolic and psycho-c\socio-spiritual systems of thought.
For many astrologers, Neptune is the ruling planet of Pisces. In Roman mythology Neptune is the god of the sea, and the deep, ocean blue color of the planet Neptune reflects this. Its glyph is taken directly from Neptune's trident. Neptune takes 165 years to orbit the sun, spending approximately 14 years (13.75) in each sign of the zodiac. Neptune was discovered in 1846.
Astrologically modern Western astrologers associate the planet Neptune with idealism and compassion, but also with illusion, confusion and deception; with religions, spirituality and mysticism, the mass media, creativity in art and music, drugs, extreme sensitivity, fantasy and imagination, psychic phenomena and altered mental states. Neptune governs hospitals, prisons, mental institutions, and any place, such as a monastery, that involves a retreat from society. Its appearance coincided with the discovery of anesthetics and hypnotism. In political terms Neptune was linked to the rise of nationalist movements throughout Europe in countries like Germany, Italy, Hungary, Ireland and Serbia, seeking independence for their nations inspired by an idealized past of legend. It was also linked to the rise of socialism and the beginnings of the welfare state. Neptune also coincided with the utopian ideals of Communism, when Marx and Engels first published 'The Communist Manifesto' in 1848.
In art, the impressionist movement began a trend away from literal representation, to one based on the subtle, changing moods of light and color. In medicine Neptune seems particularly associated with the thalamus, the spinal canal, and severe or mysterious illnesses and neuroses. Neptune is considered by modern astrologers to be co-ruler of the 12th house with Jupiter.
Nowadays modern astrologers considers Neptune the ruler of Pisces, but prior to the discovery of Neptune, Jupiter was the one considered as the ruler of Pisces, even though some modern astrologers consider Jupiter a co-ruler of Pisces (though some use Neptune as ruler of Pisces exclusively). Some astrologers do not believe that Neptune rules any particular sign, even though they may use the planet in interpretation.