The basic unit of structure and function in living organisms is the cell. Because cells are very minute, they can only be seen under high magnification with the microscope.
While some organisms are very simple, containing a single cell (unicellular), there are some organisms that are more complex (multi-cellular). In eukaryotes, cells often combine to make tissue and tissues combine to form organs. These organs may then combine (work together) to form systems.
There are 2 types of cells: prokaryotic and eukaryotic cells.
Prokaryotic cells are much simpler, and hence smaller than eukaryotic cells. They do not have an organized nucleus or the organelles found in eukaryotic cells. Examples of prokaryotes are bacteria.
Since eukaryotic cells are more complex, it means that they have an organized nucleus as well as several organelles that are not found in prokaryotic cells. Examples of eukaryotes are plants, animals, fungi and protists.
The diagram below shows the differences between a prokaryotic and eukaryotic cell.