This occurs when the side of a nail penetrates the skin causing a wound. Because there is a high bacterial population in the nail groove the wound nearly always becomes rapidly infected.
Treatment is to pack an antiseptic barrier between the nail and the wound and protect the toe from pressure from adjacent toes. This may involve removing a small piece of nail to create some space. Chronic ingrowing nails can sometimes need a small procedure where part of the nail is permanently removed.
These are areas of thickened skin which form over areas of skin stress from mechanikcal friction, pressure or shearing forces. It is part of the natural protective response of skin to prevent to maintain an intact envelope around us.
A corn is described as a circumscribed thickening of skin. It is composed of tightly compressed skin cells and may occur as an isolated lesion or as part of a callous. It is also caused by mechanical stress on skin and there may be a rotational element in its formation.