A patent family indicates a bunch of patents relating to a single patent application filed in various countries in the world having common priority date.
The important reason behind filing more than one patent application in various countries is that patent protection is country specific. A priority document can be used to take the advantage of priority over others. The patent protection is generally sought through more than one patenting authorities. Patent family is important from the point of view of global business strategy of a company.
Patent family search is an emerging filed where the searches are performed to determine technological status of a company in the desired countries.