hey cc-- you're right, a visa is a stamp you get in your passport. For some countries you need to get one before you go leave the US; sometimes you fill out a little card on the plane/train and then you go through customs (or they have you fill out the card when you get to to the customs office.) You don't need a visa for czech republic --you're american, right? I forget, sorry
You will have to go through customs though and your passport will be stamped when you arrive, and again when you depart (entering back into the EU.) I don't think you need to go through customs between EU countries; at least it's been my experience that you don't need to.
for entry requirements / visa info for american citizens. Good luck!!!