There are two types of twins: Identical twins and fraternal twins (i.e. Non-identical). Identical twins are formed when one egg splits into two after being fertilised by a sperm.

Identical twins are developed from two halves of the same fertilised egg, so they contain identical information, i.e. They will be the same sex, have similar personality, and same look. 30% of twins are identical twins.

Non-identical (fraternal) twins are formed when a woman produces two separate eggs and both eggs are fertilised two different sperms. Each will be contained within their own amniotic sac and have their own amniotic , therefore, non-identical twins are pretty much like siblings.