To be able to explore and work in space, humans have to take their environment with them because there is no atmosphere to supply the pressure and oxygen necessary to support life. Without sufficient atmospheric pressure, body fluids will start to heat up and boil. Without oxygen, we would be unable to breathe.
Spacesuits serve many functions.
Today's spacesuits are pressurized, have an oxygen supply, protect the astronaut from micrometeoroid bombardment while spacewalking, and insulate the astronaut from the severe temperature changes experienced in space. What they wear while on a mission is determined by the job they are doing. During the launch and re-entry, the astronauts wear a partially pressurized suit and a parachute pack. The suit has a helmet, gloves, and boots which all serve as protection for the astronaut. Within the suit are bladders that automatically fill with air at reduced cabin pressures.