What is an Identical Twin (Monozygotic Twin)?
Identical twins form when a single fertilized egg splits into two genetically identical parts. The twins share the same DNA because they were originally from the same fertilized egg. Because identical twins share exactly the same DNA, they will often look very similar. However, environmental factors also play a role in development and sometimes, identical twins may look very different even though they have exactly the same DNA. Identical twins are always the same sex.

What is a Fraternal Twin (Dizygotic Twin)?
Fraternal twins develop from two completely different eggs which are fertilized by two different sperm. The genetic connection of fraternal twins is no more or less the same as siblings which are born at separate times. Fraternal twins may be the same or different sexes, and they may look similar to each other or they may look very different.