This document discusses the structure and function of healthy skin and common skin conditions like acne, sun damage, and pigmentation. It explains that the skin has three layers - the epidermis, dermis, and subcutaneous layer. The epidermis contains keratinocytes that renew skin every 30-40 days and melanocytes that produce pigment to protect against UV radiation. Overexposure to the sun can cause freckles, hyperpigmentation, and depigmented patches. The dermis contains collagen, elastic fibers, and fibroblasts that provide structure and strength but degrade with sun damage and age. Active ingredients like retinoids, alpha hydroxy acids, vitamin C, and hydroquinone are used to treat acne,