Identical Twins
If an egg is fertilized by one sperm and then splits into two, the two embryos that are produced are always the same sex and are identical because they contain the same genetic material; these twins are known as monozygotic.

Fraternal Twins
Fraternal (dizygotic) twins are the result of two separate eggs being fertilized by two sperms at the same time, so that two embryos implant and develop with two placentas. The twins may look different and be different sexes.