Demeter is the queen of the fruitful earth and the greek Power of agriculture. Without her blessing, no crops may grow on the earth. Demeter will not hesitate to use this fact to blackmail men and other gods if the need exists. In gratitude for help she once received while for searching for her lost daughter Persephone (who had been abducted by Hades), she also taught the men of Eleusis the secrets of making the earth fertile. The exact nature of these secrets, called the Eleusinian Mysteries, remains unknown to this day. In her true form, Demeter is a beautiful woman, but she can change her own shape and that of others at will. She also has complete control over the weather.
Generally, Demeter is a benevolent goddess. However, if her worshipers ignore their duties, she does not hesitate to destroy their crops and send famine upon them.
Source: Legends & Lore (2nd ed.)