The distinction between a prokaryote and a eukaryote primarily lies in cell structure. Prokaryotes, such as bacteria, are unicellular organisms that lack a defined nucleus and membrane-bound organelles. Their genetic material exists freely in the cytoplasm. In contrast, eukaryotes are complex organisms (including plants, animals, and fungi) with cells that contain a true nucleus encased in a nuclear membrane, along with specialized organelles. This structural difference supports greater complexity and compartmentalization of cell functions in eukaryotic cells.