In Italy (and Europe in general), the frequency of electricity is 220 volts while in the U.S. it is 110 volts. The sockets are also a different size. For devices like your cell phone or computer, you may just need a plug adapter. An adapter doesn’t change voltage but allows a dual-voltage appliance to be plugged into the wall outlet of another country. You want to make sure with the manufacturer that your device is indeed dual-voltage. If not, you will need a bulkier transformer to actually transform the voltage.
And leave your hair dryer at home. Nearly every hotel or villa rental has an Italian one you can use.