mostarda is one of the strangest Italian delicacies. It consists of various types of fruit (grapes, pears, cherry even kiwis) steeped in a syrup made with mustard (the powdery form) diluted in water with sugar. Lots of sugar, and lots of mustard.
It is a very strange taste: the fruit retains some of its original flavor, mixed with the powerful hot-sweetness of the syrup.
It is usually served with boiled meat. I suspect it would be called a relish, if the Anglo-saxons used it.
The best mostarda is made in Cremona. Go visit Cremona, it is a beautiful place, famous for its women, its violins, the torrone. Ah, the women of Cremona ... but I digress.
As an aside, the word mostarda is a false friend, because it sounds like mustard (and moutard) while meaning something subtly different.
Ok, so now you want to know how to say mustard in Italian eh ? There you go: senape.