What are twins? Genetically what are the two types of twins that can be generated?

Twins are simultaneously generated (within the mother's uterus) offspring. Twins classify according to zygosity as monozygotic or as dizygotic twins.

Monozygotic twins, also called as identical twins, are those originated from one single fertilized ovum (therefore from one single zygote); monozygotic twins are genetically identical, i.e., they have identical genotypes and are necessarily of the same sex. Dizygotic twins, also called as fraternal twins, are those generated from two different ova fecundated by two dissimilar sperm cells; so they are not genetically identical and they are not necessarily of the similar sex.