How it works 
Generating the density function
The first step to defining the density function f=f(x,y,z) is prescribing the base or primary shape. The primary shape determines the initial values of all points in the voxel volume. A typical primary shape is the plane, where for a plane normal to the yaxis the density function f = y. The selection of the isovalue determines where the unmodified primary mesh would be generated. Density values less than this isovalue designate above a solid surface whereas values greater than this isovalue imply inside a solid.
Other primary shapes include:
Volumetric modifiers
Volumetric modifiers are functions that are applied by the user to alter density field values. Modifiers can affect the entire volume or just userdefined regions. A region is itself a volumetric modifier that nests modifiers specific to a user constrained volume. Modifiers are classed into three categories:
Surface generation
Once the values of the density volume are known an isosurface is extracted using Marching Cubes. Marching Cubes is an algorithm for extracting a polynomial of an isosurface from a threedimensional scalar field. Essentially taking the values from the eight corners of a cube (the voxel) the algorithm determines the polygons necessary to represent the isosurface passing through the cube.
References http://en.wikipedia.org/wiki/Marching_cubes
