A polish official currency is the zloty (literally "golden"). One złoty is subdivided into one hundred groszy. Most of products have price tags with an abbreviation "zł" which means the same as zloty. You can also meet PLN - a symbol of polish zloty, which is used since 1995.
Krakow as a very attractive city for tourists, has a wide range of various services. You can easily change your currency for another for e.g. USD to EUR or EUR to PLN etc. Around the Main Square you will see many currency exchange offices, in Poland so-called "Kantor". Exchange offices' owners usually show sell rate of exchange of a currency and currency purchase rate in front of an entrance, therefore are able to compare them and choose a favourable exchange rate for you.
What if you want to pay in euros?
Frankly, Poland belongs to the European Union, but not to eurozone. We use only PLN. Recently, many shops have accepted euros, therefore you can try paying in euros.