Viruses are a unique group of infectious agents whose distinctiveness resides in their simple, a cellular organization and pattern of reproduction. A complete virus particle, which is known as virion consists of one or more molecules of DNA or RNA enclosed in a coat of protein.
If we talk about the lifecycle of viruses, it can exist in two phases: extracellular and intracellular. Virions, the extracellular phase, possess few if any enzymes and cannot reproduce independent of living cells.
Virions, in the intracellular phase, viruses exist primarily as replicating nucleic acids that induce host metabolism to synthesize virion components; eventually complete virus particles or virions are released.
In summary, viruses differ from living cells in at least three ways: (1) their simple, acellular organization; (2) the presence of either DNA or RNA, but not both, in almost virions; and (3) their inability to reproduce independent of cells and carry out cell division as procaryotes and eukaryotes do.
Reference: Prescott Microbiology