The definition of life is debated, but most scientists agree on some fundamental conditions – one of the most important of which is the ability of an organism to make copies of itself. Living things are also able to transform energy from their environment, using it to maintain structural organisation – primarily for growth and reproduction.
A virus particle (or virion) does not meet the classical defining criteria for life. It is essentially a vehicle for the transfer of genetic information – DNA or RNA packaged into a protective coat – and cannot metabolise, grow or reproduce on its own. However, when viruses hijack the biological machinery of a cell, they direct it to replicate the viral genome in order to produce viral proteins and to organise the components into new virions.
Viruses evolve in response to environmental pressure, so could be considered to be extremely simplified parasites, raising the debate as to whether they do actually deserve to be classified as a form of life.