Cell are considered the basic unit of life because all life forms are composed of them. Some forms of life are made of one cell; others contain trillions.
The simplest forms of life on earth are bacteria. They are made of a single cell, and they are believed to be the first form of life to arise on earth that still exists today. While plant and animal cells are significantly more complex than bacteria, there are a number of similarities that strongly suggest that modern cells descended from bacteria.
As cells began working together, they eventually formed into distinct collections of cells. Plants and animals are the result of this evolution, and all forms of life on earth are comprised of cells with more similarities than dissimilarities. The versatile nature of cells makes them great at adapting to new roles.
Viruses are not considered cells, as they must infect a cell to reproduce and lack many structures present in other cells. When a virus is not attached or inside a cell, it is completely inert. Because of this, experts argue whether viruses should be considered a form of life. If they are alive, they represent the only non-cellular form of life known to exist on the planet.