Combustion, more commonly called "burning," is a chemical reaction between a substance and oxygen. When hydrogen burns, it reacts with oxygen according to the following chemical equation:
2H2(g) + O2(g) → 2H2O(l)
This tells us that two molecules of diatomic hydrogen gas react with one molecule of diatomic oxygen gas to form two molecules of liquid water. However, because this reaction also generates a large amount of heat, much of the water formed may actually be in the gaseous phase, and hence come off of the reaction as steam rather than liquid water.