Fire is the upward rising primary element, the counterpart of water.
Fire energy is transformational and accompanies changes of state. This fire is active, hot, and expansive, and it fills us with passion. Fire is attached to empowerment, and taking action.
Spirituality, fire represents the "divine spark" of life, and the heavenly aspects of the natural world. It also represents the kundalini energy, or spiritual force, which lies behind all Earthly manifestations of spirituality. Yogis experience "the fire of yoga", when in intense meditation. Shamans the world over demonstrate their spiritual power through mastery of fire (walking on hot coals, dancing in flames, holding hot coals in the mouth, etc.).
In a spiritual context, fire is expressed primarily via the third eye.
In Jungian psychology, fire represents intuition.
In a more mundane context, fire represents the sun (the source of all heat and life), creativity and creative energy, and virility and sexual passion (inspired by the kundalini force).
In a mundane context, fire is expressed primarily via the heart chakra.
Fire tends towards positivity.