Your cat has sebaceous glands associated with hair follicles all over her body that release an oily substance known as sebum. When released from the glands, sebum helps to waterproof the hair and maintain the skin's suppleness and health. Your cat's chin has a collection of special, larger sebaceous glands, known as the 'submental organ'. The secretion from these glands is used by your cat for scent marking. Over time you may notice this secretion as a dark, greasy material building up on items that your cat likes to mark with her face regularly.
Cat acne occurs when these submental organ glands overactive, or the hair follicle becomes blocked. This results in an accumulation of the greasy secretion on the chin hair, and the appearance of comedones (blocked hair follicles or 'blackheads'). The blocked hair follicles may become infected with bacteria, resulting in pustules. In severe cases, if many follicles are infected a deep skin infection may occur.