Cells are classified by fundamental units of structure and by the way they obtain energy. Cells are classified as prokaryotes or eukaryotes, which will be covered in more detail in the next two pages of this tutorial.
Living things are classified in six kingdoms based on structure. Within prokaryotes, which appeared 3.5 billion years ago, are the kingdoms Monera (Eubacteria) and Archaea. Within eukaryotes, which evolved 1.5 billion years ago, are the kingdoms Protista, Plantae, Fungae, Animalia.
Cells are also defined according the need for energy. Autotrophs are
"self feeders" that use light or chemical energy to make food. Plants are an example of autotrophs. In contrast, heterotrophs ("other feeders") obtain energy from other autotrophs or heterotrophs. Many bacteria and animals are heterotrophs.