using Dynamic Link Library (DLL)

Question

hi, I am doing c # and we have just started using DLL nd I have no idea what's going on. Can somebody just give me a brief background and then I want to know how to tackle this; my biggest problem now is that I dont know whether to create DLLs all the way or just one that will represent the Base class then the sub classes as normal classes.
 We are asked to create a windows Forms program that will create a Shape class(my base class) and the sub classes of rectangle, triangle and circle that will inherit most attributes from the Shape class.We are told to use DLLs!!!

Help wil be much appreciated