The last thing a cell does before it dies is maintain its distinction between inner and outer. In between the two is a plasma membrane; one of the interesting things about this membrane is that it is not tight and in fact leaks a bit in a controlled way.
The inside of the cell is called the cytoplasm and here things are tightly packed in, divided into sub-cellular compartments. The nucleus is one of these compartments; you probably remember from high school biology that there is one nucleus in each cell. It is here that we find the chromosomes that contain DNA.
Mitochondriais another sub-cellular compartment. It is the power source of the cell, the part that breathes (uses oxygen to burn food and generate oxygen). The intake of oxygen is limited to this area for a very good reason-too much oxygen can be lethal and cause premature aging and cancer.