They appear like boils. A boil or a skin abscess is a localized infection deep in the skin.
It is caused by the infection of hair follicles.
A boil generally starts as a reddened, tender area. Over time, the area becomes firm and hard. Eventually, the center of the abscess softens and becomes filled with pus and dead tissue. Finally, the pus "forms a head". This would be the whitish centre that you have described in your post.
Any break in the skin, such as a cut or scrape, can develop into an abscess if it becomes infected with bacteria. Boils are generally caused by Staphylococcus aureus, a strain of bacterium that normally lives on the skin surface. The predisposing factors are immune system disorders, diabetes, poor hygiene or malnutrition (Vitamin A or E deficiency). However, they can occur even in healthy individuals.
Most boils run their course within 4 to 10 days. You can help in resolving them by applying a warm compress. This can help alleviate the pain and hasten draining of the pus. Antibiotics are indicated only if the surrounding area is infected.