An eukaryotic organism is characterized by having complex cells that contain a nucleus and membrane-bound organelles. These organisms include animals, plants, fungi, and protists. Eukaryotic cells differ from prokaryotic cells, which lack a defined nucleus and organelles. The presence of a nucleus allows for greater genetic control and organization, enabling more complex processes such as cellular division, gene expression, and metabolism. Eukaryotic organisms often exhibit a higher level of biological complexity and diversity, facilitating specialized functions in multicellular life forms.