A springform pan is a type of cake pan from which the sides can be removed.
The side of the pan is a 'spring' that naturally opens to a slightly larger size than the bottom of the pan. The base fits into a groove that rings the bottom of side-piece. The side-piece is secured by a latch on the outer side, holding the completed pan firmly together.
This type of pan is used to bake cakes that cannot be inverted to remove them from the pan, most often cheesecakes and tortes. This design is useful for various other sorts of dishes as well, from baked Alaska to deep dish pizza.