What is the definition of fraternal twins?

Fraternal twins occur when the woman has released two eggs simultaneously, and they've both been fertilised by different sperm.

The babies born will be no more similar to each other than any other set of siblings from the same parents.

The other type is identical twins, where there's one egg that for some reason splits and form two individuals. These will usually be very much alike, as they share the exact same genetic material.