OK. To make it real simple for you – cardio exercise is any exercise that:

1. Works and strengthens your heart and lungs by revving up your heart rate and increasing your lung capacity to take in more oxygen. As a result, you keep your heart and lungs in strong and fit condition to prevent heart & lung diseases;

2. Kickstarts your metabolism higher so you’ve higher fat-burning capacity, to burn off excess body fat from your entire body, to lose weight; and

3. Builds up your cardio-respiratory (heart & lung) endurance

Good cardio exercises are many such as:

-Running
-Jumping rope
-Walking
-Biking
-Swimming
-Dancing
-Treadmill walking/jogging/running
-Jogging
-Horse riding
-Moutain climbing
-Stair climbing

This article on cardio exercise might help you out:

Do a cardio workout 30 minutes a day, that could be anything from running, swimming, jogging, jump roping, etc.
Don’t eat more then 1,200 calories a day.
If you get hungry before breakfast, lunch, or dinner eat
a vegetable or a fruit. Celery is great because when you eat it , it takes more calories for your body to burn it then their is in the celery itself so your burning calories by eating it. Cucumbers, carrots, and apples are the same. You should start losing a pound or so everyweek.
You’ll probably want to tone up too, so do 100 sit ups a day, 50 when you wake up and 50 before bed.
Its good to exercise in the morning when you first wake up because it jump starts your metabolism. Remember, drinking lots of water is a must. It keeps you hydrated, full, and your metabolism can work better. GOOD LUCK!

running is the best

Bike riding,
jumping rope,
swimminng (to name a few that I enjoy)

biking is cardio for your legs. Cardio is anything that gets your heart going so running would also be a good form of cardio for your legs.