Essentially, two things contribute to forming a pothole, moisture and traffic.  Roads are made of three different substances, pavement, which is on top, followed by sub-base and soil. Over the course of a roads life moisture from rain seeps down through the pavement into the sub-base. During the winter months when temperatures  drop, this moisture often freezes and expands. When this happens repeatedly over years, gaps under the pavement begin to form.  Vehicles pass over the weakened section of pavement until eventually the force causes the area to collapse. There we have it-a new pothole has been formed.