It is estimated that the average home contains around seven metric tons of gypsum! Gypsum has a wide range of uses, but its main use in building homes is drywall, also known as sheet rock, wall board, or plaster board.  The main reason gypsum is used is because of its chemical properties.  When gypsum is put in the presence of fire, its hydrates are released as steam.  The steam that is produced slows heat transfer and until all of the hydrates are given off will help to hold up the wooden framework of the home which allows someone trapped in a fire a few extra seconds to escape unharmed.  Drywall is built by surrounding a wet core of gypsum with large heavy sheets of paper or fiberglass mats.  When the gypsum core dries, the material then has the ability to be used as a building material.

References:

http://www.emporia.edu/earthsci/amber/go336/bray/index.htm

http://en.wikipedia.org/wiki/sheetrock

http://webmineral.com/data/Gypsum.shtml

http://www.geo.msu.edu/geo333/gypsummining.html