This article is from the Diet FAQ, by Claudia McCreary email@example.com with numerous contributions by others.
Those that cause your heart rate to increase to 65-80% of the suggested
maximum heart rate for your age group (which can be determined by
subtracting your age from 220), and which will keep your pulse at that rate
for at least 15-20 minutes continuously (i.e., "aerobic", or "oxygen-using"
exercises). Depending on your current fitness level and how hard you work,
many exercises, sports, or tasks can be aerobic in nature. You don't
necessarily have to jog or go bicycling; you can count walking, dancing, or
vigorous housework (e.g., vacuuming) as exercise so long as it causes your
heart rate to stay in that 65-80% range continuously for 15-20 minutes.
Note: While swimming qualifies as an aerobic exercise and does appear to
provide all the cardiovascular benefits of other aerobic exercises, it
appears to be less effective for weight loss than other exercises. Swimmers
tend to have higher fat levels than other athletes; this may be because fat
provides buoyancy and insulation against cool water temperatures.