The singular form is “nebula” and the plural is “nebulae”, as dictated by Latin. Fancy, but since it’s a dead language no one knows how to pronounce it properly. The term first referred to any diffuse object, including galaxies that appeared as fuzzy patches for the likes of Charles Messier (18th century) who gave us his catalog of objects that are static and are not to be mistaken for comets, which is what he was really interested in.
As optics became more refined, galaxies earned a place of their own amongst the objects in the sky, and “nebula” came to define interstellar clouds of dust and gas (helium and hydrogen mainly). All that dust and gas slowly coalesces and ends up crushed under its own weight to start nuclear fusion – stars are formed.
They generally surround stars which ionise the gas and make them shine in a discrete wavelength, specific to that particular gas, or mixture thereof. This allows the use of narrowband filters (4-6nm) that block all light except the sought wavelength.