I stole this off google as I wouldn't be very good at explaining;
1. Dichorionic/Diamniotic (di/di) twins. Di/di twins are the most common type of twins and the lowest risk. Di/di twins can be identical if the egg split very early, but fraternal twins are always di/di. In di/di twins, each twin has their own placenta and their own amniotic sac. Di/di twin pregnancies have increased risks over singleton pregnancies, but this is the best case scenario in twin world. The biggest worries tend to be going into preterm labor and making sure both babies are growing adequately.
2. Monochorionic/Diamniotic (mo/di) twins. This is what the Babies Blue are. Mo/di twins share a placenta, but each baby has it’s own amniotic sac. My doctors explained mo/di twin by saying they share a placenta and are in the same outer sac, but have a thin membrane dividing them which creates separate amnitoic sacs. Mo/di twins are always identical, though there are some references to fused placentas, which can cause some confusion. Mo/di twins are considered mid-risk in the twin world. Anytime babies share a placenta, they are at risk for twin-to-twin transfusion syndrome (TTTS), which basically means the blood and nutrients from one baby (the donor twin) are going through the placenta to the other twin (the recipient twin). In the past, developing TTTS resulted in devastating prognoses, but in recent years, thanks to development of new procedures, twins with TTTS have a much better chance of survival if it is caught early. My maternal-fetal medicine specialist told us that there is a 1 in 7 chance of mo/di twins developing TTTS.
3. Monochorionic/Monoamniotic (mo/mo) twins. This is the least common type of twins and the highest-risk type. Mo/mo twins are always identical. They have one placenta and one amniotic sac. They have the same risk for TTTS that mo/di twins have, but they also have risks associated with being in the same amniotic sac. Particularly, as the babies move around one sac, their umbilical cords can become entangled or compressed, which can result in one or both babies not receiving enough nutrients and blood.
Hope that helps explain :)