A non-corporeal lifeform, also called an incorporeal, non-physical lifeform, or formless, was a classification of species that did not maintain a corporeal form, but may or may not be capable of assuming physical form.
The properties of what made a lifeform non-corporeal were loosely defined, as they existed under many forms, such as space-dwelling lifeforms, multispectrum particle lifeforms, electromagnetic lifeforms, and included interphasic species. (VOY: "The Haunting of Deck Twelve")
In describing Trelane, Spock suggested describing him under several classifications, including "pure mentality", "force of intellect", "embodied energy", or "superbeing". (TOS: "The Squire of Gothos")
Often they were composed of various forms of coherent gas or energy. (TOS: "Wolf in the Fold", "Errand of Mercy") Of the latter, these were sometimes known by the term energy being, energy force, or energy form, and included trianic-based energy beings. (TNG: "The Bonding"; VOY: "Cathexis")
The Zetarians existed as the life force of their former corporeal existence, and were a collective composed of "the desires, the hopes, the mind and the will of the last hundred of Zetar." Together they sought out another corporeal host "through whom [they could] see and speak and hear and live out our lives." (TOS: "The Lights of Zetar")
Many had the ability to pass through solid matter or had no mass. These non-corporeals also possessed the minds of corporeal beings. (ENT: "The Crossing", "Observer Effect"; VOY: "Cathexis") During the Enterprise NX-01's mission, the crew encountered a species referred to as "Wisps". They had the ability to possess the minds of humanoids, one notable exception being Doctor Phlox. (ENT: "The Crossing") Likewise, the Organians possessed the same abilities, without exception. (ENT: "Observer Effect") The Komar of the Delta Quadrant also possessed the same abilities. (VOY: "Cathexis")
Generally, non-corporeal species did not utilize technology, compensating with intrinsic physiological capabilities. One exception was in the case of the child-being Trelane who possessed a mirror machine capable of performing matter-energy conversion. (TOS: "The Squire of Gothos") Those that possessed the intrinsic capacity to travel at warp speed, included the Companion, the dikironium cloud creature, and the Zetarians. (TOS: "Metamorphosis", "Obsession", "The Lights of Zetar") When coalesced, Q's forcefield-grid had the capabilities of traveling at warp 9.9. (TNG: "Encounter at Farpoint")
More advanced non-corporeal races, such as the Q, had evolved beyond the need for sustenance, and wielded substantial control over space and time, or possessed all around omnipotence. (TNG: "Encounter at Farpoint", "Hide and Q", "Deja Q")
Others required sustenance to survive.
While others, such as Redjac or the Beta XII-A entity, derived 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")