The two main types of cells in living organisms are prokaryotic and eukaryotic cells. Prokaryotic cells, which include bacteria and archaea, are simple, lacking a membrane-bound nucleus and organelles. In contrast, eukaryotic cells, found in animals, plants, fungi, and protists, are more complex, containing a defined nucleus and various organelles. This distinction is fundamental in biology, as it influences cellular function, organization, and evolutionary processes.