Viruses are extremely small infectious agents that invade cells of all types. Once inside another cell, viruses become hijackers, using the cells' machinery to produce more viruses. Whether viruses constitute living organisms or merely conglomerations of molecules has been a source of debate for many years.
One of the fundamental hallmarks of life is the ability to reproduce. Whether viruses have this ability is key to a debate over their status as living things. Some argue that since viruses cannot reproduce independently, they are not alive. However, similar to viruses, there are a few prokaryotes that are obligate parasites and cannot reproduce without a host. But these prokaryotes show another hallmark of life that viruses lack: growth. Once assembled, a virus does not change in size or chemical composition. They lack the machinery for producing energy to drive such biological processes. This makes them radically different from any known organism.
Viruses do, however, show some characteristics of living things. They are made of proteins and glycoproteins like cells are. They contain genetic information needed to produce more viruses in the form of DNA or RNA. They evolve to adapt to their hosts. So while it is doubtful viruses are truly alive, they are clearly very similar to living organisms.
There is more diversity among viruses than among all groups of living organisms combined. New varieties are constantly being described. It would be nearly impossible to describe all of the groups of viruses and their characteristics briefly. Instead, we will look at the characteristics shared by all viruses, such as their basic structure and general replicative cycle. Then we will discuss the features used to classify new viruses, such as shape and form of genetic material.