The Hab, short for "The Mars Lander Habitat", are a series of artificial living quarters that were constructed shortly after the Ares III manned mission to Mars landed. It is designed to house six astronauts for 31 Martian Days (Sols).
The HAB could be pressurized and depressurized using the atmospheric regulator which Mark Watney does several times in the book
Background[]
While on Mars, the Ares III crew will be living in an artificial habitat, or Hab for short. Designed for 31-days of use, the Hab will be filled with an array of supplies sent to the planet on fourteen unmanned supply missions. The unit itself is constructed of durable industrial canvas capable of sheltering its inhabitants from electromagnetic waves to minimize risks from solar radiation exposure.
Boasting 92 metres squared of floor space, three lab tables, and three airlocks, this unit is the most advanced habitat servicing NASA today. Its state-of-the-art life support systems are monitored by an automated smart system that actively regulates temperature, oxygen, and water. In addition to features such as the Oxygenator and Water Reclaimer, there are emergency backups for critical systems such as LED lighting and spare oxygen tanks as well as secondary and tertiary communication systems which communicate with the MAV.
The inside of The Hab after Airlock 1 breached.
The entire Hab is powered via solar panels arranged in a small energy farm outside the unit. Additionally, to minimise human impact on the planet, all waste and excrement is removed through a vacuum-dried waste removal system that is later collected and studied by NASA scientists to examine the effect of long-duration surface operations.
All the information you just read but in picture form. Taken from the Ares 3 Mission Guide