Command Module
The Command Module was the most vital part of any of the Saturn V Rockets.
This capulse along with the service module contained everything that the astronaunts needed to survive in their trip to the moon.

                                                                      Image: http://www.apolloarchive.com/
Both modules were needed to make the astronauts survived not only the trip but also the reentry. 
The Service module was what held all of the oxygen, batteries, and fuel and other such nessecities for making the trip.
The Command Module held the instructments needed to pilot the space craft after it had left earth orbit.  It was also the
craft in which the crew would return to earth in because it held on its bottom side a ceramic heat shield that would protect
the crew and craft from the frictional heat caused by reentry.

                                                                                                          Image:http://www.astronautix.com/craft/apolocsm.htm


Background: http://spaceflight.nasa.gov/gallery/images/apollo/apollo16/lores/as16-118-18885.jpg