The United States uses 110-120V instead of 220-230V like in Europe. The frequency is also different, 60Hz vs 50Hz. For most European electrical appliances this is not a problem. You can take your own charger with you to power your smartphone, tablet or laptop.
BUT… you will have to use an adapter to plug you charger into an American plug. The plugs used in Europe are type C, E and F and all have round pins. The American plugs are type A and B, with flat pins.
It is hard to find an adapter in the USA, so you better buy some before you leave for your trip.
Amazon.com has a lot of different adapters for American plugs. We can recommend the following adapters.