The power outlets and voltage in Europe differ from that in the US. If you want to plug in most of your electronics, all you'll need is a travel adapter that's compatible with Type C or F sockets (in use throughout Europe). Since nearly all electronics (iPhones, laptops, etc.) are compatible with both 120V (USA) and 220V (Europe) outlets, you DO NOT need a voltage converter. However, many hair dryers and hair irons are not dual voltage, so if you plan to bring your own, you'll likely need a converter (although many hotel bathrooms in Europe have a dual voltage outlet for this reason).