A diffuse nebula is an irregular cloud interstellar matter composed of gas and dust within which stars can be born.

Diffuse Trifid Nebula is an emission and reflection Nebula

Emission nebula

Close to very hot UV emitting stars, gas clouds (hydrogen) are ionized and emit a red light which is characteristic of diffuse emission nebulas.

The Nebula behind the Horsehead Nebula is an emission Nebula

Reflection nebula

Diffuse reflection nebulas, with a dominant blue, do not emit light but reflect the light of other stars.

M78 is a reflection nebula in the constellation Orion.

Other deep sky objects: