Is Nebula the result of supernova explosions?

Answer 1

There's many different kinds of nebula; ring nebulae, or planetary nebulae are the direct result of a star exploding, expelling much of it's mass in huge rings or spheres. Sometimes, radiation and heat from the star's leftover core makes the remnants glow, such that we can see it here on Earth.

Other nebulae, the Orion nebula for example, is a conglomeration of gas & dust, where stars are actually forming.

Answer 2

Some are and some are not. Here's a short article with some decent links to more info.

http://en.wikipedia.org/wiki/Nebula