Epidermoid cysts
Cysts are cavities filled with fluid or air. They can occur just about anywhere. They are harmless and easy to remove. Sometimes you can get them in the outer layer of the skin called "epidermis" where they occur in areas with a lot of sebaceous glands. This is why they most often occur on the scalp, face and upper body. They can also penetrate deeper tissues if they become large. This can cause a painful inflammation where a lot of pus often forms.