Aerobic exercise is a sustained effort and can include running, elliptical machines, swimming, biking, or anything that uses large muscle groups over a period of time — usually longer than fifteen minutes.
How do you know if you're running or biking (or whatever) fast enough to be "aerobic?" Simple: Check your pulse. And try to keep in your "aerobic heart range." If your pulse is too fast, slow down. If it's not fast enough, go faster.
Here's how to find your ideal heart rate during aerobic exercise:
Subtract your age from 220. That number is your maximum heart rate (number of heartbeats per minute).
Your heart rate during exercise should be between 60% - 80% of your maximum heart rate. Make it 60% if you're a beginner. As you get in better shape, you can eventually work out at 80% of your maximum heart rate.
The best way to check your pulse is to put two fingers on the side of your neck and count the number of beats for ten seconds. Multiply that by six for your number of heartbeats per minute.
Many exercise machines have heart rate monitors built into them, which makes it easier. You can also get a heart rate monitor that fits on your wrist.
If you're a beginner, check your heart rate every ten minutes or so. If you've been doing it for a year or more, you can check it every twenty minutes.
If all this sounds terribly complicated, it really isn't. After figuring out your ideal heart rate, you'll know it and after measuring it many times, you'll get to know the feel of being in your aerobic range.
If you'd like to have more motivation to exercise, read this: Exercise Motivation.
For more information about aerobic heart rates, I recommend these books:
Precision Heart Rate Training
High School Healthy Hearts in the Zone: A Heart Rate Monitoring Program for Lifelong Fitness (this is a good one for beginners)
The Heart Rate Monitor Guidebook to Heart Zone Training
Total Heart Rate Training: Customize and Maximize Your Workout Using a Heart Rate Monitor