True. Gunpowder also known as black powder is a mixture of charcoal, sulfur and potassium nitrate (saltpeter). The charcoal and sulfur act as fuels and the saltpeter acts as an oxidizer. Sugar could also be used as the fuel instead of charcoal. A traditional ratio of the ingredients would be about 15:3:2 potassium nitrate, charcoal, and sulfur by weight.
A word on how it's made:
“The second kind of flying fire is made in this way. Take 1lb of native sulfur, 2lb of linden or willow charcoal, 6lb of saltpeter, which three thing are very finely powdered on a marble slab. Then put as much powder as desired into a case to make a flying fire or thunder. Note: The case for flying fire should be narrow and long and filled with well-pressed powder. The case for making thunder should be short and thick and half-filled with the said powder and at each end strongly bound with iron wire.” –Marcus Graecus. Liber Ignuim, ca. 1280 AD
Not all of the recipes in the Liber Ignium can be trusted but this one is pretty much dead on.
References:
Partington, James Riddick. A History of Greek Fire and Gunpowder, The Johns Hopkins University Press, 1998
Dunn, Kevin M., Caveman Chemistry, Universal Publishers, 2003 ISBN 1-58112-566-6