Traditionally, in Italy Tip is just meant to be a sign of appreciation, and most times just symbolic. Waitstaff are supposed to have their own salary just like any other restaurant employee. Even so, however, it gets a bit confusing here as well. While in a basic restaurant you may just not leave any tip, or tip with a couple of Euros just to say "thanks, you were nice", if you are in a more expensive restaurant, tradition is to tip more, and, in some occasions, up to 10%. Some would say that % also depends on the number of people in the party (the more, the higher the tip).
Which is why waitstuff here tends to love US tourists who don't know or happen to fail in adapting to local customs.

But, I have to say that quality of service in Italy is, on average, not nearly comparable to that in the US. So far, US waiters are in my personal experience the best trained, nicer, and most careful I've met. Not that here servie is particularly bad (and I find it improving, fortunately). Just, not as good, at all. Most times waiters's kindness or care is part of their own personal people skills. They don't get trained, they don't get told what customers like and dislike, and there's just a less developed culture of customer care.