Infill is the amount of material (expressed in percentage) that is used to fill the interior geometry of the part
The infill properties that have the greatest changes are the geometry shape and infill percentage. These properties will alter the structural integrity, how much axial or lateral stress it can bare and therefore affect its weight proportionally .
From the image above some patterns of different geometries are printed as well as the infill density (on the right) with a single pattern ranging from 0 to 100%