Anaerobic exercise is a physical exercise intense enough to cause lactate to form. Oxygen is not present with anaerobic exercise and glycogen is used as fuel.
Once all the glycogen has been depleted (usually in about two hours) one can expect to hit the proverbial wall.
Anaerobic exercise is mostly used by athletes in non-endurance sports to promote strength, speed and power and by body builders to build muscle mass.
Examples include sprinting and weight lifting.