Most credit cards are NOT free to use abroad. Most have either a service fee or transaction fee for spending in a shop, hotel, restaurant etc and/or something called currency loading where they take the VISA/MASTERCARD rate for the day and apply a percentage to that to effectively make your transaction more expensive. Typically this is up to a couple of per cent.
I would recommend that if you already have a credit card you check with your issuer what their fees and charges are for oeverseas usage. If they have any and you intend to use a credit card overseas then I would consider applying for a card that is specifically for designed for overseas usage. This will be far cheaper and make your money go further. The link here takes you to some of the cards that will have no transaction fees and no currency loading fees. They may also have no ATM fees for withdrawing cash, however interest on credit card cash advances kicks in from the day you take the money out, with the exception of the Sainsburys Gold card who only apply it if yu don't pay your balance off in full each month.
Hope that this helps you.