The mitochondria are the energy factories of the cell. This double membraned organelle takes glucose and combines it with oxygen to produce carbon dioxide, water, and ATP (chemical energy, usable by the cell).
The purpose of the mitochondria is to power the cell. These organelles take in glucose and oxygen and break them down to provide ATP (chemical energy). This process is called cellular respiration. They preform almost the exact opposite function of the chloroplasts, which provide (through the process of photosynthesis) all the materials necessary for cellular respiration. In a plant cell, these are complimentary. The chloroplasts develope the materials necessary for providing the cell with energy, and the mitochondria use the materials to manufacture it.
Interesting note: This organelle, along with chloroplasts, are almost cells in and of themselves. They have a double-membrane with their own DNA, and reproduce by splitting themselves.
The mitochondria are in the cytoplasm.