I use the acronym of MR. GOSEA to help recall the characteristic of all living things: maintain homeostasis, reproduce, grow, organization (cells --> tissue --> organ --> organ system --> organism), respond to stimuli, use or make energy and adapt.
Viruses are made of a protein capsid that houses genetic material. Therefore they are not made of cells, which are considered the smallest unit of life. For this reason, viruses are not considered living.
Viruses are also not considered to be living because they cannot reproduce on their own. Viruses must invade a preexisting cell and use that cell's genetic to undergo protein synthesis in order to replicate additional body parts that later come together to make more viruses. The cell then becomes so full of new viruses that it explodes. All the viruses that were created within the host cell are then free to invade other cells. This is called the lytic life cycle (lyse = to explode).