The two main types of cells in biological organisms are prokaryotic and eukaryotic cells. Prokaryotic cells are simple, unicellular organisms that lack a defined nucleus and other membrane-bound organelles; bacteria are a prime example. They typically have a cell wall, cell membrane, cytoplasm, and genetic material in a singular circular DNA strand. In contrast, eukaryotic cells are more complex, containing a nucleus that houses their genetic material and various organelles such as mitochondria and the endoplasmic reticulum. Eukaryotic cells can be unicellular or multicellular, forming organisms like plants, animals, and fungi. Understanding these cell types is fundamental in biology and medicine.