The main distinction between prokaryotic and eukaryotic cells is the presence of a nucleus and membrane-bound organelles. Prokaryotic cells, such as bacteria, lack a nucleus; their genetic material is dispersed in the cytoplasm. In contrast, eukaryotic cells, found in organisms like plants and animals, possess a true nucleus containing their DNA and various organelles that perform specialized functions. This structural difference is a foundation for the complexity of multicellular organisms.