According to Wikipedia:

“Hermeticism, is a religious and philosophical tradition based primarily upon writings attributed to Hermes Trismegistus (“Thrice Great”). These writings have greatly influenced the Western esoteric tradition and were considered to be of great importance during both the Renaissance [14th to the 17th century] and the Reformation [16th to the 17th century].”

Alchemy—The Operation of the Sun—is not simply the changing of physical lead into physical gold. It is an investigation into the spiritual constitution, or life of matter and material existence through an application of the mysteries of birth, death and resurrection.

The various stages of chemical distillation and fermentation, among them, are aspects of these mysteries, that, when applied quicken Nature’s processes in order to bring a natural body to perfection. This perfection is the accomplishment of the Magnum opus (Latin for Great Work).
Astrology—The Operation of the Moon: Hermes claims that Zoroaster discovered this part of the wisdom of the whole universe, astrology, and taught it to man. In Hermetic thought, it is likely that the movements of the planets have meaning beyond the laws of physics and actually holding metaphorical value as symbols in the mind of The All, or God. Astrology has influences upon the Earth, but does not dictate our actions, and wisdom is gained when we know what these influences are and how to deal with them.
Theurgy—The Operation of the Stars: There are two different types of magic, according to Giovanni Pico della Mirandola’s Apology, completely opposite of each other. The first is γοητεια, Goëtia, black magic reliant upon an alliance with evil spirits (i.e. demons). The second is Theurgy, divine magic reliant upon an alliance with divine spirits (i.e. angels, archangels, gods).
Theurgy translates to “The Science or art of Divine Works” and is the practical aspect of the Hermetic art of alchemy. Furthermore, alchemy is seen as the “key” to theurgy, the ultimate goal of which is to become united with higher counterparts, leading to the attainment of Divine Consciousness.”

Pasted from <http://en.wikipedia.org/wiki/Hermetism#Late_Antiquity>