The most obvious difference between viruses and bacteria is size! To us, a bacteria is microscopic but to viruses the size of a bacterium is massive. Bacteria can be thought of as the size of a van in comparison to viruses, which can be considered the size of the average suitcase. Size is not the only reason a bacterium can be seen as a van, it can also somewhat act like a van.
A van is a machine consisting of fuels, pumps, etc. and thus a bacterium is also an enclosed machinery. However, a virus is like a suitcase because they don't encompass in a sort of "machinery"; instead they have simple genetic material that can potentially invade the bacteria and control its genetic information. In other words, a virus (a suitcase) holds the "drivers manual" for the bacterium (the van). But what's more important, how can we fight a virus that has infected a bacterium?