Types of aerobic exercise are:

1. Walking.
2. Running.
3. Cycling.
4. Swimming.
5. Cross country skiing.
6. Stair climbing.
7. Skipping.
8. Dancing.
9. Rowing.
10. Roller skating.
11. Basketball.
12. Water aerobics.
According to recent research just 10 minutes of aerobic exercise 3 times a day 5 days a week will help your fitness levels tremendously. There is also the concept that one does not need to kill oneself exercising, especially if you are just starting on a fitness program. It is the duration of the exercise rather than the intensity that matters.
In the initial stages you should be able to hold a conversation with someone while you are exercising. This is low intensity training.
Once you are comfortable with that level you can step it up one, to moderate intensity, where you are breathing harder and conversation is a little more difficult but your breathing soon returns to normal once you stop.
The next level will have you breathing hard and deep but you will not be totally out of breathe. You will not be talking much!
These 3 levels are usually enough for anyone not trying to train for a sporting event! If you are comfortable with any level, feel you should be doing more but are not yet ready to step it up a level, just increase the duration, eg. Walk for 60 minutes instead of 30.