Some credit cards are fine for use internationally, but others charge a fee (often 3%) for purchases made overseas. This is a wholly-unjustified charge whose purpose it to enrich the banks.
This is a handy list of some cards that do charge a fee:
Careful Which Card You Use Abroad: These Credit Cards Charge a Foreign Transaction Fee
Obviously, if you own several credit cards and are traveling (or making online purchases from a foreign company), you will want to use the cards with no fee.