Credit cards (and debit cards) are usually the preferred option for making purchases overseas because the currency conversion (the actual exchange rate you see in the papers or on the news) is usually at the wholesale rate (or 1% over the commercial bank rate) and not at the retail rate you would pay for changing cash at the airport, hotel, or bank.
Not only this, using cards is just as secure abroad as when you are at home (for example, if used fraudulently). You also don’t have to carry cash around, taking it out and putting it back into your purse or bag each time, and you don’t have to worry about counting out or receiving the correct change.
Credit and debit cards are accepted almost everywhere in the developed world, and Visa tends to be accepted more than the others. Bear in mind, however, that the budget hotels, hostels and cafes used by budget travellers may not accept them.
If you do use them you can also postpone paying the bill (with a credit card) and take advantage of any differences in exchange rates between the purchase date and the money being converted (if you travel to countries with ever rising inflation).
Disadvantages of paying with a credit card include the fact that some card issuers (and this applies to debit cards too), will charge you a fee for foreign currency conversions, perhaps between 2 and 4%, usually 3% of the transaction amount.
Some debit card purchases attract an additional flat fee on top of this foreign exchange fee each time you use your debit card (check with the card issuer).
You can, however, find card issuers that charge very little or nothing at all. For example, check out Capital One in the US, or Nationwide, Lombard Direct and the Post Office in the UK.
You can compare UK card charges for overseas use 
here.
Remember to find out any daily /weekly/ monthly limits your bank imposes on transactions using your card.
Finally, don’t forget to inform your bank or other financial institution of your travel plans, where you are going and for how long, as automated fraud systems can sometimes block access to you funds at inopportune moments. If possible, give them a mobile phone number where they can contact you 24 hours a day and failing that, find out their customer service number (24 hour if possible), so you can call them if there are any problems.
