Complex sentence: It is a mixture of sulfur, charcoal, and potassium nitrate (saltpeter)--with the sulfur and charcoal acting as fuels, while the saltpeter works as an oxidizer.
Split into simpler sentences: It is a mixture of sulfur, charcoal, and potassium nitrate (saltpeter). The sulfur and charcoal act as fuels, and the saltpeter is an oxidizer.