What is the difference between a pimple, a pustule and a papule?

According to Webster's Ninth New Collegiate Dictionary:

pimple 1 : a small inflamed elevation elevation of the skin : PAPULE; esp. : PUSTULE 2 : a swelling or protuberance like a pimple

papule : a small solid usu. conical elevation of the skin

pustule 1 : a small circumscribed elevation of the skin containing pus and having an inflamed base 2 : a small often distinctively colored elevation or spot resembling a pimple or blister

"...Small, red, solid bumps (called papules) and pus-filled pimples (called pustules) may appear on the skin. Because these appear similar to acne, rosacea is sometimes called acne rosacea or adult acne. However, unlike acne, there are no blackheads..." source

"...Acne vulgaris is associated with plugging of the ducts of the oil glands, resulting in blackheads and pimples on the face and sometimes also the back, shoulders or chest. Rosacea seems to be linked to the vascular network of the central facial skin and causes redness, bumps, pimples and other symptoms that rarely go beyond the face..." source

"Pimples appear as the disease progresses. These may appear as papules (small, red, solid bumps)." source