The terms twins and twins are often used incorrectly, as they are not the same thing.
Two siblings are twins when they come from the same egg and the same sperm. What happens is that the embryo resulting from fertilisation splits in two. Therefore, the two babies will have the same genetic information.
Twins, on the other hand, come from two eggs fertilised by two different sperm. Each fertilisation will give rise to one embryo and one future baby. Twins share only half of their DNA, so they would be two siblings who are the same age.