An astronaut is a living human being that primarily explores and observes space using special instruments designed for a special purpose. He/She is generally trained for what field of astronomy that person wants to go in.
For Multiple reasons. 1) There is no air, therefore the astronaut must wear the suit, or rather the suit's helmet to breathe. 2) It protects him from physical damage when handling volatile or dangerous substances/ objects.
Astronauts have oxygen tanks for the air they need. They also have special kind of food that allows them to still have their vitamins and nutrients. This food is generally served in tubes because of the lack of gravity. Likewise, they have special cups for the beverages they drink.