The cells that lack a well defined nucleus surrounded by a nuclear membrane are called prokaryotic cells (pro means primitive;karyon means nucleus).
Examples of prokaryotes are bacteria and blue- green algae.
The cells that have a well-defined nucleus are called eukaryotic cells. Organisms that have such cells are called eukaryotes (e means, true; karyon means nucleus).
Examples of eukaryotes are animals, plants and fungus etc.