Eukaryotic cells are complex cells that contain a nucleus and other membrane-bound organelles. They are found in organisms such as plants, animals, fungi, and protists. The presence of a nucleus allows for the organization of genetic material, which is stored in the form of DNA. Eukaryotic cells can be unicellular, like yeast, or multicellular, like humans. They are characterized by their ability to perform various functions, including energy production, growth, and reproduction, through specialized structures within the cell. This complexity distinguishes them from prokaryotic cells, which lack a nucleus.