Pigmentation refers to the colouring of skin. Healthy skin appears normal in colour, but when there is a problem the colour may change. Skin gets its colour from melanin, a pigment that is produced by special cells called melanocytes; these occur the basal layer of the epidermis.
When the melanocytes become unhealthy or damaged the production of melanin is disrupted. Most pigmentation problems are localised and affect patches of skin, but they can be widespread and affect the whole body.
If the skin darkens it is called hyper-pigmentation, and if it gets lighter it is called hypo-pigmentation.