silica gel, having molecular formula (SO2) is amorphous and porus. Silica has a tridimensional framework of alternating Silicon and Oxygen atom. Thhe voids here may be filled with any gas/liquid/vacuum.When the voids are filled with vacuum, the silica gel is called Silica Xrogel.
Silica Xerogel has a strong affinity forwater molecules therefore is used as a dissicant. Silica Xerogel is thus put up with dry substances to absorb the moisture present around it.
Silica Gel is mistaken to absorb moisture but on the molecular level the silica molecule carries out the process of ADSORPTION rather than ABSORPTION. (In Absorption the liquid is carried out to the bulk of the solid whereas in Adsorption the liquid sticks on to the surface pores of the solid.)