Everyone is required to have a valid passport (or ID card for EU citizens) to enter the Czech Republic. Citizens of EU and EEA (Europe Economic Area) countries do not need a visa for any type of visit. Citizens of Australia, Canada, Israel, Japan, New Zealand and the USA can stay for up to 90 days without a visa; other nationalities can check their visa requirements on www.czech.cz. Visas are not available at border crossings or at Prague's Ruzyně airport; you'll be refused entry if you need one and arrive without one.
Non-EU citizens who want to stay in the Czech Republic for more than 90 days must apply for a long-term visa, employment visa or residency permit; get details from your nearest Czech embassy, and apply at least four months in advance.
Visa regulations change from time to time, so check www.czech.cz.