Papule- a group of many small papules and microcomedones is almost invisible but we all must have touched a friend casually and felt that “sandpaper” like feel. That is how a papule is like. It is extremely small, (5 millimeters or less) and is slightly inflated above the skin. It is caused by localized cellular reaction to the process of acne.
Pustule- this is what we usually refer to a pimple. It is like a circular eruption on the surface of the skin, containing pus that comprises a mixture of white blood cells, dead skin cells and bacteria. A pustule that forms on the sebaceous gland usually has a hair in the centre. When squeezed or picked, it may eject blood also. So it is recommended to remain untouched as it heals (generally), and leaves no marks.
Macule- when an acne lesion heals, it leaves behind a reddish brownish mark with a distinct outline, which stays in for days or weeks. These marks are called macule. After a violent outbreak of pustules, the face has a reddish inflamed appearance, which is nothing but collective macules.
Nodule- resembling a papule, a nodule is solid, irregularly or dome shaped skin lesion. but the difference between a papule and a nodule is the characteristics of a nodule to cause inflammation, extension into the deeper layers of the skin, like the endodermis, where it might cause destruction of the tissue, leading to scars. Another feature associated with the nodule is extreme pain. Unlike other mild forms of acne, nodular acne cannot be treated by anything except a drug called isotretinoin which is available as ointments.