Acne vulgaris

simple term is Acne as described Acne Vulgaris by Wikipedia.Org

An inflammatory skin condition affecting sebaceous gland ducts. It occurs most frequently in adolescent years due to hormonal imbalance occurring at the time of puberty, however, it is not limited puberty. It can affect the face, chest and back. Excessive oil, comedones, papules, pustules and scarring may be present depending on the individual and severity of the condition.
It is caused when pores or follicles become blocked with sebum and skin cells, but the sebaceous gland continues to produce sebum. Due to the blockage, this trapped sebum cannot escape anaerobic bacteria within the pore of follicle. The bacteria then multiplies and the structure becomes infected leading to the formation of pus.