Wrinkles are folds or creases in the skin that develop with ageing or body mass loss, due to exposure to sunlight, de-hydration etc. The extracellular fibres in the derma layer of the skin are injured with repeated extensions and compressions of the skin. While our body repairs these injuries, some of the broken fibres are not regenerated. They are instead replaced by long collagen fibres. When these long fibres accumulate, they make the skin inelastic and loose that leave a big fold on the skin.
Wrinkles may be temporary or permanent. The formation of both these forms is just the way it is explained above, however in the case of permanent wrinkles, when the long collagen breaks in a compressed state, it is replaced by short collagen fibres. These short fibres limit the expansion of long fibres and the long fibres are left in a permanently folded state. Wrinkles are a major cause of ageing.