Poland is a part of the Shengen Area. Visas for the EU citizens and citizens of some other countries are not required. For details consult links below.
http://www.msz.gov.pl/en/travel_to_poland/visa/visa
http://ec.europa.eu/dgs/home-affairs/what-we-do/policies/borders-and-visas/visa-policy/index_en.htm