When you send bitcoins it is advisable to include a small transaction fee, most wallets will calculate an appropriate fee automatically (rarely more than a few pence), this provides the miners an incentive to confirm the transaction quickly.
Additionally, if you are making payment to a merchant via a Bitcoin payment processor, the processing company may charge a fee. This fee is often hidden within the exchange rate and, depending on the agreement between the merchant and processor, can include the cost of the merchant converting the bitcoins back to their preferred currency.
This is not a cost of paying with Bitcoin but rather a cost of using a intermediary to process the payment. Unfortunately most merchants insist you use their choice of intermediary, instead of accepting Bitcoin directly. Watch out for hidden fees!
See the blog post “The True Cost Of Paying With Bitcoin” for a more detailed explanation.