Question

What is a planetary nebula? Why do many planetary nebulae appear as rings?

Answer 1

Planetary nebulas, such as the ring nebula (m57) have distinct ring or cylinder shapes, and are the result of a runaway expansion of the star, which is way less intense than a (super)nova, which would lead to a far less organized cloud.

Explanation:

The material that is ejected forms a ball-shaped shell of finite thickness. If we look to the center, we see only two thin layers of that shell (front and back). If we look more to the sides, we see a much thicker layer, because we look 'into' it at a very oblique angle.
This will give the impression of a ring.