When mammals have two babies at once, each of them is a twin. If you have a twin brother or sister, the two of you are always the same age.
There are identical twins, who have the same DNA, and fraternal twins, who are more like two siblings who happen to be born at the same time. It's more common to describe humans as twins than other animals. The adjective twin can also describe a pair of things, like the twin chocolate cakes you serve at your birthday party or the "Twin Cities" in Minnesota, Minneapolis and St. Paul.