explanation of gaussian surface.

Gaussian surface is a r eal or  an imaginary surface which is arbitrarily chosen for the application  of a Gauss's theorem(such as to  calculate the electric field of the sphercial symmetrical bodies, line chage and  a very thin infinite plane sheet). Whenever we want to calculate the electric field around  in side or out side of the symmetrical body we draw the gaussian surface  ,  like for spherical shell we  draw the   imaginary spherical  gaussian surface, for infinite line charge  we  draw the cylinderical gaussian surface and for the  thin infinite plane sheet we draw the pile box penetrating the  thin infinte plane sheet. and then do required calculations.