The basic concept of witchcraft is that human behavior is a mirror of fundamental laws of nature. In nature, there is a continuous cycle of birth, death, and rebirth occurring. By observing these cycles in nature, one can obtain a better understanding of oneself.
The human psyche can be considered to have a masculine and a feminine side. These are symbolized in witchcraft by the Goddess and the God.
Some books would tell you that witchcraft is feminine in nature, with the major deity of the religion being a female Goddess. In some forms of witchcraft there is indeed a tendency toward worship of the Goddess alone (Dionysian witchcraft being a very apt example of this), but in it's original form, witchcraft is about worship of the masculine and feminine as equal and complimentary forces.
For this reason it is sad that many mainstream books on the subject tend to deal with the Goddess in great detail and yet do not give as much attention to the God.
The cycles of nature can be reflected in the story of the God and Goddess and how their lives affect each other. The Sabbats and festivals of the Wiccan religion are celebrations of the story of the God and the Goddess as a mirror image of the seasons and the changing face of the world.
In general, witches tend to believe in reincarnation. This follows the concept of birth, death, and rebirth.
One important point is that witches DO NOT worship the devil. We are peaceful and nature loving, not evil.