Chroma keying deletes pixels by color: shoot against saturated green, key the green out, and the subject floats on whatever background the mix provides. Green won because sensors resolve it cleanly and skin contains little of it.
Live rigs key in real time: dancers over generative backgrounds, a keyed camera feed as a VJ layer. Quality hangs on lighting, since even keying needs an even screen, and on spill control where green bounces onto subjects.
The keyed result is transparency, which in digital terms means an alpha channel: chroma key manufactures alpha from color, where formats with true alpha carry it natively.