elemental : A lower order of spirit being that exists as the life-force in the natural world, whose task it is to maintain harmony. In occult lore, elementals are said to govern minerals, plants and animals; the four elements of earth, air, fire and water; the planets, stars, signs of the zodiac; and hours of the day and night. They are ruled by angels. Most are viewed as benevolent, though some are malicious or trickster-like in behavior.

The Neoplatonic Greeks grouped elementals according to the four elements of life. Earth elementals are gnomes, ruled by the angel Ariel; air elementals are sylphs, ruled by Cherub; water elementals are undines, ruled by Tharsis,; and fire elementals are salamanders, ruled by Nathaniel or Seraph. In the fifth century, Proclus added a fifth group which lives beneath the ground, and in the eleventh century, Psellus added a sixth group, the lucifugum, which means "fly-the-light."

The term "elemental" is applied to a broad range of spirit beings, also including elves, who live in the woods and along the seashore, and household spirits such as brownies, goblins, bogles and kobolds, and fairies.

Elementals appear in a variety of forms. Some are humnan-like, such as the dwarfish gnomes. Sylphs appear as butterflies and undines as waves. Some are more like angels, and area amorphous shapes of white light surrounded by flowing, colorful auras of energy. British Spiritualist Grace Cooke said that elementals enjoy human company, can understand human speech, and respond to music. They have their own karmic evolutions, progressing toward higher forms of life.

British medium Geraldine Cummins channeled information about elementals, purportedly from the deceased Frederic W. H. Myers, one of the founders of the Society for Psychical Research in London. In automatic writing, "Myers" described elementals as the essence which emanates from forms of life such as trees and plants, and which coalesces into a form perceived by the human mind as a sprite.
source for additional reading/information:

Hall, Manly P. Paracelsus: His Mystical and Medical Philosophy. Los Angeles: The Philosophical Research Society, 1964.
Steiner, Rudolph. The Influence of Spiritual Beings Upon Man. Spring Valley, N.Y.: Anthoposophic Press, 1961.