A non-corporeal lifeform is a species composed of various forms of coherent gas or energy. (TOS: "Wolf in the Fold", "Errand of Mercy") Some non-corporeal entities absorb and utilize energy directly from their environment. (TNG: "Lonely Among Us"; ENT: "Zero Hour")
Others, such as Redjac or the Beta XII-A entity, derive sustenance from the emotions of humanoid lifeforms. The Dikironium cloud creature of Tycho IV forcibly extracted the corpuscles of iron-based blood and fed on that. (TOS: "Wolf in the Fold", "Obsession", "Day of the Dove")
Kes becomes non-corporeal
Generally, non-corporeal species do not utilize technology, compensating with intrinsic physiological capabilities. Some non-corporeal races have apparently evolved beyond the need for sustenance, and wield substantial control over space and time. (TNG: "Encounter at Farpoint")
In early 2374, Kes became a non-corporeal lifeform (VOY: "The Gift") after USS Voyager's encounter with Species 8472 in the Delta Quadrant, and the growth of her telekinetic, or psionic abilities.