Gauss' Law applies to the electric flux through a closed surface. In this case the area vector points out from the surface.
Gauss' Law states that the net electric flux through a closed surface is proportional to the charge Q enclosed by the surface.
Net flux = Q/eo
The constant eo is known as the permittivity of free space, and is given by:
eo = 1/4pk = 8.85 x 10-12 C2 / Nm2
The more charge enclosed, the greater the flux through the surface. The net flux is positive if the net charge enclosed is positive, and negative if the net charge enclosed is negative. If there is no net charge enclosed by a surface the net flux is zero - any field lines entering the surface must leave the surface somewhere else.
As we will see, when the geometry of a situation is highly symmetric, Gauss' Law can be a very powerful method of calculating electric fields.