On board were astronaut "Gus" Grissom, (the second American astronaut to fly into space) astronaut Ed White II, (the first American astronaut to "walk" in space) and astronaut Roger Chaffee, (a "rookie" astronaut on his first space mission).
After the crew had been working on training for several hours, a fire broke out inside the capsule. Because there was a lot of oxygen inside, and oxygen burns very easily, the fire spread very fast. People outside the capsule could not open the hatch fast enough, and the astronauts died.
After the experts figured out what had happened, they fixed a lot of problems, and later, the Apollo program continued and on July 20, 1969, Neil Armstrong on the Apollo 11 mission was the first person to step on the moon.