To survive in space, humans require access to the basic life support items necessary to survive on Earth: food, breathable air, potable water, a means of removing waste and adequate shelter from extreme environmental conditions. In addition, humans require a means of transiting from Earth to space and back.
Humans can travel to and from outer space by means of spacecraft, but aerospace technology has not advanced sufficiently enough to allow humans to live in space permanently. Without adequate protection from the environmental extremes of outer space, a human can die within seconds of exposure to high-energy radiation, extreme cold, vacuum effects and numerous other threats.
Unlike the habitable Earth, outer space is a completely hostile environment for the human body, so spacecrafts must have a self-sufficient life support system on board. Even so, the supplies brought on spacecrafts are carefully rationed to last for the duration of a trip to outer space. Beyond this time, spacecrafts must either return to Earth or receive replenishment supplies from other spacecrafts.
Spacecrafts function not only as vehicles, but complete life support systems as well. The spacecraft itself shields humans from the environmental extremes of outer space. Spacesuits offer a similar form of protection to space travelers in the event of a need to perform extravehicular activities, or space walks, for experimental or spacecraft maintenance requirements. However, space walks are temporary ventures outside of the spacecraft, and spacesuits have a finite life support capacity.