The Czech Republic is part of the EU. But it still has its own currency - the crown, called " Kč " or "CZK".
In many places (especially petrol stations, supermarkets and big shops), you can, of course, pay by standard payment cards (VISA, Euro Master). For other purchases, it is also good to have crowns in cash.
Do not accept just any price (at the first exchange office on the border) if you are not sure about a good rate. You also pay a lot of fees in many currency exchanges - so always ask how much is the net / in your hand amount!
The most secure way you can exchange money is at the accommodation reception, where there are no hidden fees to surprise you.