A pergola is a structure which is designed to create a cool, shaded area beneath, which can also be used for recreation.
A pergola is an open framed construction, similar to a patio but without solid roofing.
A pergola can consist of just posts and simple framework to provide a structured space to sit or work under, or it can have a more interlaced framework either for decoration or for growing vines over.
More often a pergola is used to grow vines which create a shady space to sit under in summer. When the vine leaves fall in winter it allows the sunshine in while still maintaining a sense of cosiness. Used in such a way it can also often also be referred to as an Arbour.