Areobics

High impact usually involved jumping or climbing and is high impact on jointsie:jogging, step aerobics, etc.. Low impact you can do the same exercises without the jumping...low impact on your body...ie: swimming. Hope that helped..

High impact aerobics are usually also high intensity aerobics. However the proper definition is aerobic exercises that place high levels of stress to your joints and bones. Not recomended for people with joint problems or weak bones.
Fear not you don't need to perform high impact to get high intensity. You can use a low impact aerobic exercise such as Swimming/cycling and still work at a high intensity if u wish for all the benefits.