Eukaryotic cells have a well-defined nucleus containing the cell's genetic material (DNA), which is enclosed by a double membrane called the nuclear envelope. 

This membrane separates the genetic material from the cytoplasm, which is the fluid-filled space inside the cell that contains various organelles, including membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes. 

Unlike prokaryotic cells, which lack a nucleus and membrane-bound organelles, eukaryotic cells are more complex and evolved from prokaryotic cells through a process of endosymbiosis.