The Cullen Crest is a detail added in the film that doesn't appear in the books. Every member of the Cullens wears it on some sort of jewelery. It shows an arrow, three shamrocks, a lion, and a hand above the lion. If you are wondering about the meaning or the need of such a crest - stop right now, because this is Twilight. If something is shiny and beautiful and cool, it doesn't need any meaning or explanation!