Eukaryotic cells are complex cells that have a defined nucleus, which contains their genetic material. These cells are found in a wide variety of organisms, including animals, plants, fungi, and protists. The presence of membrane-bound organelles, such as mitochondria and endoplasmic reticulum, allows eukaryotic cells to perform specialized functions efficiently. In contrast to prokaryotic cells, which are simpler and lack a nucleus, eukaryotic cells can be much larger and more organized. This complexity enables eukaryotic organisms to develop into multicellular forms, like humans and trees, showcasing a vast range of life on Earth.