Spontaneous Generation suggests complex things form spontaneously.
Abiogenesis suggests populations of simple things can evolve.
To elaborate a little more, Spontaneous Generation was proposed by Aristotle more than two millennia ago. Scientists stopped taking it seriously after Louis Pasteur performed several demonstrations in the 1800's showing that sterilization prevented complex life from spontaneously forming. By contrast, Abiogenesis proposes a gradual process. It was proposed after the theory of evolution was well-understood, and depends heavily upon it. Abiogenesis is currently the leading scientific explanation for the origin of life on Earth, and several semi-plausible mechanisms have been discovered that may have played a role in its occurrence.
