Viruses are submicroscopic particles. These cannot be seen by microscope. Virus is considered as a link between living and non living organisms. As it can remain inactive for several years outside host cells and become active and show signs of living organisms once they come in contact with the living host. This property of viruses makes them a tremendous killing machine as most of them are pathogenic in nature.
Viruses are even smaller than bacteria unicellular cells.
Viruses are not even a complete cell. As they lack all kinds of protein mechanieries and consist of only genetic material covered by a protein code. When they come in contact with a living host they insert their genetic material into the host cell and use the host protein mechanism for the production of protein of their interest. They multiply inside the host cell and make numerous copies of themselves when they come out from the host cell after killing it, many times more virus cells are present to infect many more host cells.
Virus cells have either DNA or RNA as genetic material. DNA is the universal genetic material but viruses that use RNA as genetic material are called retrovirus.
On infecting a host cell virus cell only inserts its genetic material to the cell. Which either bind to host genetic material and start replicating with it or replicate and translate independently to make more and more viral proteins and viral genetic material.
Viruses are named based on their genetic structure to facilitate the development of diagnostic tests, vaccines and medicines. Virologists and the wider scientific community do this work, so viruses are named by the International Committee on Taxonomy of Viruses (ICTV).
Prions are even smaller than virus partials; they are just abnormal infectious proteins that can be transmitted from one to another.