Viruses can only live as pathogens. Outside another living organism they can do nothing, but once inside a plant or animal cell they can grow and multiply.
Viruses generally consist of a single loop of DNA surrounded by a coat of protein. A few have RNA instead of DNA.
A virus uses its own DNA to control what its host cell does. The viral DNA gets used by the host cell to make lots of the viral protein coat. It then replicates the viral DNA. Eventually the host cell bursts, releasing lots of new viruses to infect more host cells.