Planetary nebulae are shells of gas thrown out by some stars near the end of their lives. Our Sun will probably produce a planetary nebula in about 5 billion years. They have nothing at all to do with planets; the terminology was invented because they often look a little like planets in small telescopes. A typical planetary nebula is less than one light-year across.

Emission nebulae are clouds of high temperature gas. The atoms in the cloud are energized by  light from a nearby star and emit radiation. These nebulae are usually red because the predominant emission line of hydrogen is red. Emission nebulae are usually the sites of recent and ongoing star formation.