Taken from: http://www.nhs.uk/chq/Pages/2419.aspx?CategoryID=52&
How do I know if I'm exercising moderately or vigorously?
Moderate intensity aerobic exercise is where you're working hard enough to raise your heart rate and break into a sweat. You're working at a moderate intensity if you're able to talk but unable to sing the words to a song. Not much use if you only know instrumentals!
Vigorous intensity aerobic exercise is where you're breathing hard and fast and your heart rate has increased significantly. If you're working at this level, you won't be able to say more than a few words without pausing for a breath.
Not sure that it is quite as beneficial as it seems to suggest, but anything that makes you feel better and feel healthier, and actually makes you generally healthier overall is worth doing IMHO.
dave