Identical twins are conceived from one fertilised egg. This egg then separates into two babies. The chromosomes are completely identical as they come from the same egg and sperm. They are assigned the same sex at birth and share genetic characteristics, such as eye and hair colour.
Non-identical ones are generally known as fraternal twins. If the mother releases two eggs at the same time, a different sperm will fertilise each. They are often as alike as their siblings, but not identical. This is the same for twins of a different gender.
There are also conjoined twins. Typically they are joined at the chest and abdomen, although it’s not unknown for them to share a head. They also share at least one vital organ.