The three main parts of a cell are the plasma membrane, the region containing the DNA and the cytoplasm. However, not all cells have exactly the same basic parts. There is a difference between the structures of eukaryotic and prokaryotic cells.
The plasma membrane separates the cell from its environment and regulates the substances that flow in and out of the cell. Additional structures, such as the flagella and cilia, may be found on the cell's external surface. All cells have DNA, but the region containing the DNA is called the "nucleus" in eukaryotic cells. Prokaryotic cells do not have a nucleus. The DNA is simply found in the cytoplasm or the semi-fluid substance within the cell. Other organelles may also be found within the cytoplasm, including ribosomes, endoplasmic reticulum, Golgi bodies, vacuoles and mitochondria.