This article is from the Fitness FAQ, by Jeff Gleixner (firstname.lastname@example.org) with numerous contributions by others.
Aerobic means oxygen. Your muscles are working in an oxygen rich
state. After 12 minutes it causes your body to produce FAT-burning
According to Covert Baily, an aerobic exercise is one that fits the
following four requirements.
A. Is steady, nonstop.
B. Lasts twelve minutes minimum.
C. Has a comfortable pace.
D. Uses the muscles of the lower body.
An easy way to measure if your training too hard/fast is a simple
talk test. If you can't carry on a conversation without huffing
and puffing for breath you need to slow down. If you can carry on
a conversation without stopping every now and then for a breath
then you need to go a little faster. You have to be doing some
work. Your heart rate and your breathing needs to be elevated.
Examples: walking/running, cycling, rowing, stair climbing, dancing.