Sensu is a folding fan made of paper on a bamboo frame, usually with a decorative picture, design, or calligraphic character on it. It makes a popular foreign tourist souvenir especially in Kyoto.It is also used as an important stage prop in traditional Japanese dance and rakugo comic storyteller.In addition, it is a must item in tea ceremony.
On the other hand, uchiwa is a round, flat paper fan with a wooden or plastic handle. It is often used to advertise many things such as new products, shops and some events. Sometimes we get uchiwa in some event or on the street. Compared with sensu, uchiwa has a more casual image.
I have an impression that young students fan themselves by uchiwa while older people carry sensu in their bags.