Mitochondria are the organelles in which the most important part of the cellular respiration occurs, i.e., the ATP production.
Mitochondria are organelles bounded by two lipid membranes. The inner membrane infolded to the interior of the organelle forming cristae that defines the internal space known as mitochondrial matrix and where mitochondrial DNA (rafDNA), mitochondrial RNA (ratRNA), mitochondrial ribosomes and respiratory enzymes can be found.
Mitochondria are found in eukaryotic cells in high number and are also more abundant in those cells that use more energy, like muscle cells. Because mitochondrion have their own DNA, RNA and ribosomes. They can even has the ability to self-replicate.