Twins are two children produced by the same pregnancy. They can either be monozygotic meaning identical or dizygotic meaning fraternal.
When a set of twins are of separate sexes, they are by definition dizygotic twins, as they are no more related than any other sibling.
Monozygotic twins are the result of a fertilized embryo splitting itself into two identical specimens. As such, Fred and George Weasley were monozygotic twins, as their identical appearance was often capable of fooling their own mother.