Mitochondria (the purple guys in the sketch above) are tiny, little organelles scattered around the cell body. They are the remnants of ancient bacteria. Back in the early days of life formation a bargain was struck between our cells and bacteria. Bacteria would be admitted into the cell body in exchange for performing the vital function of generating energy to power the cell. You can think of mitochondria as the power plants of the cell - ATP production.
Why should you care? Because there is growing consensus that mitochondrial dysfunction is at the heart of chronic disease and aging. Mitochondrial health is a function of quantity (number of mitochondria per cell) and quality (how efficient they are). In the sketch above there are only 3 mitochondria, but cells can grow more.
A recent study, The Pleiotropic Effect of Physical Exercise on Mitochondrial Dynamics in Aging Skeletal Muscle, shows that exercise improves both the quantity AND quality of mitochondria in muscular tissue. Furthermore, the combination of endurance and strength training improves mitochondrial health better than performing only one of those activities.
Here's the take-home message. If you want to stay young, hit the weights... and also hit the cardio. A training regimen that incorporates both will be most effective in maintaining your vitality.