Aerobics is hard exercise done to music, usually in a class: lots of stepping, jumping, and moving your arms so that your heart and breathing work hard for a long time. That is the point of the older science word aerobic: the body is working with oxygen, with air. Though actually, people now stretch aerobics to mean almost any sweaty music class, even when the teacher calls it cardio or dance fitness. It was a huge word in the 1980s (bright clothes, home videos), and it still names that old-style class. In more careful talk, aerobic exercise is the general idea (a long run counts); an aerobics class is the lesson with the teacher and the beat. You go to aerobics; you do aerobics. It is informal-to-neutral, not a doctor's technical term.