Answer:
Whenever two babies are born at the same time, they are considered to be twins. If the two babies resulted from a single zygote that became two genetically identical fetuses, the result is identical twins (who will be very similar in appearance and who will always be of the same gender). If, however, the two babies are the result of two separate zygotes which happened to be formed at about the same time by the fertilization of two separate ova, then the result is fraternal twins (who resemble each other only to the extent that any sibling resembles another sibling, and who are not necessarily of the same gender, which is to say, they can be brother and sister).