According to Wikipedia (borrowing, as it's a pretty good description of this wondrous veggie!):
Romanesco broccoli (also known as Roman cauliflower, Broccolo Romanesco, Romanesque cauliflower or simply Romanesco) is an edible flower bud of the species Brassica oleracea. First documented in Italy, it is chartreuse in color. Romanesco has a striking appearance because its form is a natural approximation of a fractal. When compared to a traditional cauliflower, its texture as a vegetable is far more crunchy, and its flavor is not as assertive, being delicate and nutty.
The Romanesco has been grown in Italy since the 16th century.