Twin-to-twin transfusion syndrome occurs specifically in identical twins sharing the same placenta. It is estimated to occur in 5-10% of identical twin pregnancies. In twin-to-twin transfusion syndrome, the twins share not only the same placenta but some of the same circulation. This allows the transfusion of blood from one twin (the donor) to the other (the recipient). The donor twin becomes small and anemic, and the recipient twin becomes large and overloaded with blood.
I lost one of my triplets to this syndrome - at the time there wasn't much information on it.  The above link is a great source for information if you or anyone you know is experiencing this now.