A pergola is a structure which is designed to create a cool and shaded area beneath the pergola which can 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 more interlaced framework either for decoration or for growing vines over.
More often a pergola is used to grow vines over to create a shady space to sit under in summer, while in winter when the vines leaves fall it can let in the Winter sun while still providing a sense of cosiness underneath. Used in such a way it can also often also be referred to as an Arbor or Arbour.