The main difference is in their nature: virus is non-cellular form of life which is an intracellular parasite in the living organism. It doesn't function out of cells. It do not have its own protein synthesis system. All its proteins, carbohydrates and lipid components are the products of the host cells. Bacteria is an autonomous cellular form of life which can live and successfuly& reproduce independently of other organisms. It can have the various possible interrelations with other bacteria, algae, animals etc, not only being as parasite.