Mitochondria are membranous organelles that are responsible for producing ATP.
Mitochondria consist of two membranes - an inner membrane and and outer membrane. The inner membrane has a series of folds called cristae. This increased surface area allows the mitochondria to produce a large amount of ATP (since the molecules involved in making ATP molecules are embedded in the inner membrane).
The more work that a cell does, the larger the concentration of mitochondria in the cell. For instance, skeletal muscle cells have a large number of mitochondria and although sperm contain mitochondria, the number of mitochondria in skeletal muscle cells is greater than the number in sperm.
Another interesting thing about mitochondria is that when a person begins training using aerobic exercise, such as long-distance running, the number of mitochondria in the skeletal muscle cells increases.
Mitochondria contain their own DNA, which allows them to make some of their enzymes.