What do you mean 'how' and 'why'? There are two basic types of twins - monozygotic, commonly referred to as identical twins and dizygotic, referred to as fraternal twins. There are also other rare and unusual types of twins, such as polar body twins. The specific cause of twinning is still a mystery. Identical Identical, or monozygotic, twins develop from one fertilized egg (mono=one, zygote=egg). This egg divides into two individuals who will share all their genes in common. No one knows what causes the egg to divide in half. These twins are genetic clones of each other. They will therefore always be the same sex and have identical features, eye and hair color. Their handprints and footprints are similar, but the fingerprints are different. This type is what most people think of when they think of twins. Fraternal Twins Fraternal, or dizygotic, twins develop from two eggs (di=two, zygote=egg) that are fertilized separately by two different sperm. This usually happens when the mother produces more than one egg at ovulation. These two fertilized eggs develop separately and have their own unique genes. They are no more alike than single brothers and sisters genetically. Dizygotic, or fraternal, twins may be the same sex or opposite and may appear similar or look completely different. They may have different hair and eye colors and be different sizes. Some may not even appear to be related! More Than Two... Other multiple types are possible, such as triplets which consist of three babies; quadruplets (or quads) which are four babies; quintuplets (or quints) which are five babies; sextuplets which are six babies; and septuplets which are seven babies. These are all referred to as higher order multiples or supertwins. Higher order multiples can be any combination of the two basic twin types. Original text: http://www.nomotc.org/library/twinning_facts.html