The term aerobic exercise actually means “with oxygen.” When you perform aerobic exercise, your muscles generate the energy you need using a complex series of chemical reactions that involve oxygen. That’s also the reason your breathing speeds up (to get more oxygen) and your heart begins to race (to pump that oxygen-rich blood to the muscles that need it).
Strength training is also called anaerobic exercise, which means “without oxygen.” That’s because the processes that turn oxygen into energy don’t work fast enough to keep up with anaerobic exercise. Instead, your body makes up the difference with a different set of reactions that use the energy stored in your muscles as glycogen. These energy reserves are limited, which is why your muscles can’t sustain anaerobic exercise for long before they seize up.
Incidentally, when you do aerobic exercise, you’re primarily using your slow-twitch muscle fibers. When you do strength training, you’re putting your fast-twitch fibers to work.
Source of Information : Oreilly - Your Body Missing Manual