identical twins

Because there's a split after fertilization. They share the same exact DNA from the start, 1 egg, 1 sperm, therefore they will be 'identical'. (there are some differences, and for that, you'll be needing to check on search engines and in books).

A quick view in simplest form is as follows

What sturgis said, however not all twins are identical. They can also come from separate eggs which were both fertilized.

I wonder if we should explain triplets and other multiples.

Enuff already!