What is the difference between Simple and Covalent Bonding?

A simple covalent bond exists between isolated molecules like H2,N2,HCl,H2O,etc In molecules like SiO2, each Si atom is joined to 4 O atoms and each O atom is attached to 2 Si atoms.Thus SiO2 exists as a giant polymeric molecule.Another example is Diamond in which each carbon is tetrahedrally attached to 4 other C atoms.it also exists as a giant polymer.