It's not just aerobics. Biking, running, jogging, treadmill, stepper, etc... is considered cardio. Its something you do to get your heart rate up and pump more oxygen into your blood and get a good sweat going. Weight training can also have a cardio effect if you take short rests in between sets, this will also get a sweat going.
Remember keep asking questions but also do some searching for your question first, it might have been answered already. AJ