Eukaryotic cells are complex cells that contain a nucleus and other membrane-bound organelles. They are found in organisms such as animals, plants, fungi, and protists. The nucleus houses the cell's genetic material, which is organized into chromosomes, allowing for more advanced functions and regulation of cellular activities. These cells are generally larger than prokaryotic cells, which lack a nucleus. Eukaryotic cells can be unicellular, like yeast, or multicellular, like humans and oak trees. Their structure allows for specialization, enabling different cell types to perform specific functions within an organism.