With the possible exception of viruses, every form of life on Earth either is a cell or is composed of cells. All cells come into existence through the activity of other cells.

All cells can be classified as prokaryotic or eukaryotic. All plants, animals, fungi, and protists either are single eukaryotic cells or are composed of eukaryotic cells. All prokaryotes are made of single cells.

Eukaryotic cells have most of their DNA contained in a membrane-lined nucleus, whereas prokaryotic cells have their DNA in a localized region but do not have a nucleus. Eukaryotes have specialized structures, called organelles, while prokaryotes do not. All prokaryotes are single-celled, whereas many eukaryotes are multicelled.