Liquid Hydrogen--the Fuel of Choice for Space Exploration (click here)

Despite criticism and early technical failures, the taming of liquid hydrogen proved to be one of NASA's most significant technical accomplishments. . . . Hydrogen -- a light and extremely powerful rocket propellant -- has the lowest molecular weight of any known substance and burns with extreme intensity (5,500°F). In combination with an oxidizer such as liquid oxygen, liquid hydrogen yields the highest specific impulse, or efficiency in relation to the amount of propellant consumed, of any known rocket propellant....

Got that? Liquid Hydrogen + an oxidizer like Liquid Oxygen = propellant

Back in the day, it was originally liquid oxygen. But, when there was a deadly explosion killing three astronauts, the oxygen was mixed with hydrogen.