Cardio, or aerobic exercise, is any exercise that increases your heart rate and keeps it elevated for a sustained period of time. Your heart rate doesn't have to stay at the same rate the whole time, however. High Intensity Interval Training is a great example of a cardio workout in which your heart rate will fluctuate...and that's ok!
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 into your Target Heart Rate Zone. 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 are 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!
If you're just beginning a cardio program, there are 3 main points to think about.
If you are a beginner, focus on time first and don't worry so much about getting faster or working harder. That will come later as you continue to gain fitness.
The CDC Physical Activity Guidelines state that adults should get, at the very least, 30 minutes of moderate physical activity 5 days a week. Start small with 10-20 minutes of activity 3 days a week and work up from there. Once you are comfortably reaching the minimum guidelines then you can work on increasing the intensity.
You don't need to allow as much recovery time between cardio sessions like you do with strength training, however, you still don't want to overdo it.
The order in which you do your cardio and strength training will depend on your goals and what feels comfortable to you. Keep in mind that cardiovascular exercise will induce fatigue that could compromise your form during strength training. You could adjust your schedule to do separate workouts in a single day or alternate which comes first but ultimately, you need to do what works for you and any order is better than not doing it at all!