What are aerobic and anaerobic exercise?

In one line

Aerobic exercise is long, steady exercise where muscles use oxygen to make energy; anaerobic exercise is short, intense exercise where muscles make energy fast without oxygen.

What are aerobic and anaerobic exercise?
As in 'I only did cardio today'
At the gym, treadmills and indoor bikes are called 'aerobic', and short, hard exercise is called 'anaerobic'. The word 'oxygen' is hidden in these names because of how muscles make energy. This post explains what the two words mean and how they mix in real exercise.
In one line
With oxygen for long, without oxygen short and hard
Aerobic exercise is exercise where muscles use oxygen to make energy while large muscles move rhythmically for a long time. Anaerobic exercise is very short, very hard exercise where muscles make energy quickly without oxygen.
First, muscle energy
Muscles draw power from a substance called ATP
When muscles move, they take energy from a substance called 'ATP'. But the ATP stored in muscle lasts only a few seconds. So during exercise, muscles keep making new ATP, and there are two ways to do it: one that uses oxygen and one that does not.
The way that uses oxygen
Slow, but makes a lot for a long time
The chemical reactions in the body that break down nutrients to get energy are called 'metabolism'. Breaking down sugar and fat with oxygen to make ATP is 'aerobic metabolism'. It is slow, but from the same sugar it gets about 9 times more ATP than making it without oxygen.
The way without oxygen
Fast, but does not last long
Breaking down energy stores and sugar kept in the muscle without oxygen to make ATP is 'anaerobic metabolism'. It is very fast but does not last: at full effort it can keep going for only a little over 1 minute. 'Anaerobic' does not mean holding your breath; it means this reaction does not use oxygen.
Aerobic exercise
Large muscles, rhythmic, for a long time
Exercise that moves large muscles rhythmically for a long time, like walking, running, cycling, swimming, or dancing, is aerobic exercise. During it, the heart beats faster and breathing gets harder. US health authorities count being able to talk but not sing as moderate intensity.
Anaerobic exercise
Very hard exercise lasting seconds to about 1 minute
Very hard exercise that ends within a few seconds to about 1 minute, like sprinting or lifting a very heavy weight, is called anaerobic exercise. It runs mainly on ATP made by anaerobic metabolism. That is why it cannot be kept up for long at the same intensity.
The two are mixed
The longer it lasts, the bigger the oxygen share
In real exercise the two types of metabolism run together; only the mix changes. When all-out exercise lasts about 1 minute 20 seconds, the two are half and half. The longer it goes beyond that, the bigger the share of aerobic metabolism.
Common misconception
Does aerobic switch on only after anaerobic runs out?
Many people believe that anaerobic metabolism is used first when exercise starts, and aerobic metabolism takes over only after it runs out. But the two run together from the start. Even in 10 seconds of all-out effort, about 1/10 of the energy came from aerobic metabolism.
Summary
With oxygen or without it
Aerobic exercise is longer exercise where muscles use oxygen to make energy; anaerobic exercise is short, hard exercise where muscles make energy fast without oxygen. The two types of metabolism do not switch on in turn; they always run together and only the mix changes. Related terms: ATP, lactic acid, HIIT.