Poland is a European Schengen nation, so travelers from the US and other western countries do not need a visa. A 90-day entry permit will be granted upon arrival. Americans can explore http://travel.state.gov/visa/ for more information.
Health and Safety
Poland is an extremely safe country, especially in comparison to other large European nations like France, Italy and Germany. Of course, there are places to avoid in some of the larger cities, but common sense always prevails. Some of the nightlife districts can get a bit rowdy at times, and street brawls are not uncommon, but there is only a small chance you'll run into trouble vacationing in Poland. You should be aware though that homosexuality in Poland (as with other Eastern European states) is considered taboo.
Roads are getting better as modern infrastructure continues to improve, but there are still some poorly maintained highways and streets outside of the larger cities. Driving laws are strictly enforced in Poland, so you may find yourself in trouble if you're well over the speed limit or driving without a license.