Prokaryotic cells are distinguished from eukaryotic cells primarily by their cell structure. Prokaryotic cells, such as bacteria, lack a nucleus and other membrane-bound organelles, meaning their genetic material is not enclosed within a nuclear membrane. They are typically smaller and simpler in structure. In contrast, eukaryotic cells, found in animals, plants, fungi, and protists, contain a defined nucleus and various organelles, allowing for compartmentalization of cellular functions. This distinction is fundamental to various biological processes and organism classification.