In Western culture the snake is the great seducer: in the paradise story, it is the snake that entices Eva to take a bite from the forbidden apple, leading to the Fall. And in the Gilgamesh epos, it is a snake who steals immortality from Gilgamesh. But besides being a symbol of evil - and even Satan - , the snake is also a symbol of fertility and regeneration - because it can shed its skin. The Sumerian fertility god Ningizzida - who also became the god of healing - was depicted as a serpent with a human head. And as is well known, the Greek medicine god Asclepius carried a serpent-entwined staff.
In Asia, the snake was close to the divine dragon. In Indian mythology we have the Nagas, great dragon-like serpents, who possessed many magical powers and guarded great treasures. In Buddhism, Nagas were believed to be both water-dwellers, living in streams, and earth-dwellers, living in underground caverns. They also guarded Mt. Sumeru, the Axis Mundi. In the legend of the Buddha's life we encounter a naga called Mucalinda - when Sakyamuni sat meditating under the Bodhi tree, a heavy rain started and Mucalinda with his seven snake heads formed a sort of umbrella above the Buddha's head to protect him from the elements.
In Japan, the serpent is especially associated with the syncretic Benzaiten, the goddess of everything that flows: water, words, and music. She is the main deity of the shrines on islands as Enoshima and Chikubushima and is often represented with a snake coiled around the rock on which she is seated. In Japanese legend, the snake is also a symbol of a woman's jealousy: in the famous story about Kiyohime, the jealous woman transforms herself into a serpent and coils around the temple bell in which her fugitive lover has hidden, literally "frying" him with her passion.
Perhaps because of the "Naga treasures," the snake is also associated with money and profit - on New year cards we often find it accompanied by gold coins.
Japan knows many snakes (as anybody who has hiked in Japan's forests can attest to); they are an ingredient in traditional medicine. Dangerous is the mamushi, the pitviper, whose bite leads to several deaths each year (another venomous snake is the habu, found on Okinawa).
The Year of the Snake is associated with the earthly branch symbol 巳 (mi), and this is how it is written on New Year's cards.