Cardio, or aerobic exercise, is any exercise that increases your heart rate and keeps it elevated for a sustained period of time.
Why do cardio?
If you're thinking exercise DVD's are your only option for cardiovascular fitness at home, you'd be surprised. If you're lucky enough to have a machine like a treadmill, elliptical or stationary bike then you can build a great cardio program around that. If not, there are still plenty of ways to get a good workout. Just get creative...and in some cases, even silly!
The key is to pick something you enjoy that will get your heart rate up. There is no one particular activity that is better than another. The best exercise for you is the one you'll enjoy and stick with.
If you are planning to train for a specific event such as a 5K, then you'll want to plan your cardio based on preparation for that but, in any case, variety, or cross training, is important. Even if you're using only one type of machine or exercise, you can still add variety. You don't have to run the same 3 mile loop or do the same elliptical program every day! You can change effort levels or do intervals or change any aspect of the exercise just a little. Don't overthink it, just do something!