They convert oxygen and nutrients into adenosine triphosphate (ATP). ATP is the chemical energy "currency" of the cell that powers the cell's metabolic activities. This process is called aerobic respiration and is the reason animals breathe oxygen.
Mitochondria are bounded by a double membrane. Each of these membranes is a phospholipid bilayer with embedded proteins. The outermost membrane is smooth while the inner membrane has many folds. These folds are called cristae. The folds enhance the "productivity" of cellular respiration by increasing the available surface area.
The double membranes divide the mitochondrion into two distinct parts: the intermembrane space and the mitochondrial matrix. The intermembrane space is the narrow part between the two membranes while the mitochondrial matrix is the part enclosed by the innermost membrane. Several of the steps in cellular respiration occur in the matrix due to its high concentration of enzymes.
Mitochondria are semi-autonomous in that they are only partially dependent on the cell to replicate and grow. They have their own DNA, ribosomes and can make their own proteins. Similar to bacteria, mitochondria have circular DNA and replicate by a reproductive process called fission.
The inner membrane selects over what materials are allowed through it and it is known that active transport mechanisms involving translocase enzymes are responsible for the movement of ADP and ATP across it.
Mitochondria use respiration to brake down high energy molecules such as sugars and store that energy as ATP (produced from ADP & a phosphate)
The raw materials used to generate ATP are