Jenova is an extraterrestrial lifeform, composed of cells that carry its will and power even when separated. Jenova originally crashed into the Planet and began infecting the residing Ancients until they almost died out; it was later excavated kwa Shinra and mistakenly thought to be an Ancient.
Jenova is the Calamity from the Sky, the one who, thousands of years ago, rode to the planet on a meteor, which caused the North Crater upon impact. She is an extraterrestrial who almost ruled this whole planet and almost wiped out the entire Cetra race, when the people took her destiny away from her. She, her cells and her energy, helped me become myself. She also happens to be my mom. :) Haha.