This article is from the Aerobics Fitness FAQ, by firstname.lastname@example.org (Robles) with numerous contributions by others.
The word aerobic literally means "with oxygen" or "in the
presence of oxygen." Aerobic exercise is any activity that
uses large muscle groups, can be maintained continuously for
a long period of time and is rhythmic in nature. Aerobic activity
trains the heart, lungs and cardiovascular system to process and
deliver oxygen more quickly and efficiently to every part of the
body. As the heart muscle becomes stronger and more efficient, a
larger amount of blood can be pumped with each stroke.
Fewer strokes are then required to rapidly transport oxygen
to all parts of the body. An aerobically fit individual can
work longer, more vigorously and achieve a quicker recovery
at the end of the aerobic session.